CSS Left align div regarding a center align parent div - html

Question

My issue is certainly very easy regarding that i am new in css.
I will show you first a picture of my issue :
I am tryin to put the grey to the left of the content alignes with my other div. The issue is that my grey div is always centered.
.row {
display: flex !important;
justify-content: center;
align-items: center !important;
}
.payment-title-div {
color: #fff;
background: #1282a2;
width: 70%;
}
.u-left {
padding-top: 0.5em;
}
.return-div {
padding-left: 15%;
}
.payment-div {
background-color: lightgray;
width: 50%;
height: 400px;
}
<div class="row">
<div class="payment-title-div">
<h1 class="u-left size12">
<svg class="lock-icon" xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="currentColor" class="bi bi-shield-lock" viewBox="0 0 16 16">
<path d="M5.338 1.59a61.44 61.44 0 0 0-2.837.856.481.481 0 0 0-.328.39c-.554 4.157.726 7.19 2.253 9.188a10.725 10.725 0 0 0 2.287 2.233c.346.244.652.42.893.533.12.057.218.095.293.118a.55.55 0 0 0 .101.025.615.615 0 0 0 .1-.025c.076-.023.174-.061.294-.118.24-.113.547-.29.893-.533a10.726 10.726 0 0 0 2.287-2.233c1.527-1.997 2.807-5.031 2.253-9.188a.48.48 0 0 0-.328-.39c-.651-.213-1.75-.56-2.837-.855C9.552 1.29 8.531 1.067 8 1.067c-.53 0-1.552.223-2.662.524zM5.072.56C6.157.265 7.31 0 8 0s1.843.265 2.928.56c1.11.3 2.229.655 2.887.87a1.54 1.54 0 0 1 1.044 1.262c.596 4.477-.787 7.795-2.465 9.99a11.775 11.775 0 0 1-2.517 2.453 7.159 7.159 0 0 1-1.048.625c-.28.132-.581.24-.829.24s-.548-.108-.829-.24a7.158 7.158 0 0 1-1.048-.625 11.777 11.777 0 0 1-2.517-2.453C1.928 10.487.545 7.169 1.141 2.692A1.54 1.54 0 0 1 2.185 1.43 62.456 62.456 0 0 1 5.072.56z"/>
<path d="M9.5 6.5a1.5 1.5 0 0 1-1 1.415l.385 1.99a.5.5 0 0 1-.491.595h-.788a.5.5 0 0 1-.49-.595l.384-1.99a1.5 1.5 0 1 1 2-1.415z"/>
</svg> <strong>Paiement sécurisé</strong>
</h1>
</div>
<div class="return-div">
<a class="titre" href="books"><svg class="arrow-icon" xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="currentColor" class="bi bi-chevron-double-left" viewBox="0 0 16 16">
<path fill-rule="evenodd" d="M8.354 1.646a.5.5 0 0 1 0 .708L2.707 8l5.647 5.646a.5.5 0 0 1-.708.708l-6-6a.5.5 0 0 1 0-.708l6-6a.5.5 0 0 1 .708 0z"/>
<path fill-rule="evenodd" d="M12.354 1.646a.5.5 0 0 1 0 .708L6.707 8l5.647 5.646a.5.5 0 0 1-.708.708l-6-6a.5.5 0 0 1 0-.708l6-6a.5.5 0 0 1 .708 0z"/>
</svg>Retour à l'annonce</a>
</div>
<div class="payment-div">
</div>
</div>
Any indication ?
BR

Answer 1

Well I've copied your code and I didn't get the exact same from your picture, but I'd recommend this changes in your CSS code:
.row {
display: flex !important;
flex-direction: column; /* set the direction as column (one element above another) */
align-items: flex-start; /* align at start (left) */
}
.payment-title-div {
color: #fff;
background: #1282a2;
width: 100%; /* I've changed this to 100% */
}
.u-left {
padding-top: 0.5em;
}
.return-div {
/* padding-left: 15%; remove this padding */
}
.payment-div {
background-color: lightgray;
width: 50%;
height: 400px;
}
You can also check this Flexbox Guide to understand better how to work with flex boxes, It's really awesome am I right?