<div class="parent"></div>
.parent {
display: block;
width: 100%;
height: 300px;
background-color: yellow;
margin: 0 70px;
box-sizing: content-box;
}
.parent {
display: block;
width: calc(100% - (2 * 70px));
height: 300px;
background-color: yellow;
margin: 0 70px;
box-sizing: content-box;
}