Blog post issue on 7.1

Site URL: https://www.hvmc.sg/blog/engaging-our-youth

I used this code to make the first image on the post a bit wider than the rest of the post but I think the :first-child selector is messing up the layout of the whole page. Does anyone know what I could do instead to target the first block on a blog post?

.blog-item-wrapper {
overflow: hidden;
  .sqs-block:first-child {
left: 50%;
transform: translateX(-50%);
width: 70vw;
}
}

Try this class name

.blog-item-wrapper .span-12>.sqs-block:first-child