Use the align-items property with value flex-end to align flex items on the bottom.
Example
You can try to run the following code to implement the flex-end value
<!DOCTYPE html>
<html>
<head>
<style>
.mycontainer {
display: flex;
height: 300px;
background-color: #5DADE2;
align-items: flex-end;
}
.mycontainer > div {
background-color: #F2F3F4;
text-align: center;
line-height: 60px;
font-size: 30px;
width: 100px;
margin: 5px;
}
</style>
</head>
<body>
<h1>SOCIAL SECURITY Numbers</h1>
<div class = "mycontainer">
<div>SS1</div>
<div>SS2</div>
<div>SS3</div>
<div>SS4</div>
<div>SS5</div>
<div>SS6</div>
</div>
</body>
</html>