<div class="block"></div>
.block{
width: 300px;
height: 100px;
background: #5C72FA;
position: relative;
}
.block:before{
content: '';
position: absolute;
bottom: -20px;
left: 0;
border: 20px solid transparent;
border-left: 150px solid #5C72FA;
}
.block:after{
content: '';
position: absolute;
bottom: -20px;
right: 0;
border: 20px solid transparent;
border-right: 150px solid #5C72FA;
}