@media (min-width: 576px) {
  #topx {
    width: 100%;
    height: 100vh;
    background: url("../../assets/img/modelX-mobile.jpg") center / cover no-repeat;
  }
}

#topx {
  width: 100%;
  height: 700px;
  background: url("../../assets/img/modelX.jpg") center / cover no-repeat;
}