.big-box {
    background: red;
    width: 400px;
    height: 200px;
    border-radius: 10%;
    position: absolute;
    top: 50%;
    left: 50%;
    position: absolute;
    transform: translate(-50%,-50%);

    } 

.big-box:before{
    content:"";
    position: absolute;
    width: 0;
    height: 0;
    border-top: 25px solid transparent;
    border-bottom: 25px solid transparent;
    border-right: 50px solid red;
    margin: 25px 0 0 -25px;
        
        }