body { width: 90em; margin: 0 auto; 

}
#parent {
  display: flex;
}
  #narrow {
  background-image: url("");
  background-repeat: no-repeat;
  background-position-x: center; 
  border: 1px solid #000000;
  border-radius: 10px;
  margin: 0px;
  width: 530px;

  /* Just so it's visible */
}

  #wide {
  flex: 1;
  border: 1px solid #000000;
  border-radius: 10px;
  margin: 10px;
    width: 200px;
  /* Grow to rest of container */
  background: #999999;
  /* Just so it's visible */
}