function  video(){
 if(document.myfrom.mybox2.checked==true)
     {
       window.document.getElementById("videocard_num").disabled=false;
       var a3 = window.document.getElementById("videocard_a3").value;
       var a5 = window.document.getElementById("videocard_a5").value;
       var a12 = window.document.getElementById("videocard_a12").value;
       var total = 3.3 * a3 + 5 * a5 + 12 * a12;
       window.document.getElementById("videocard_total").value = total;
    }
  else
      {
       window.document.getElementById("videocard_num").disabled=true;
       window.document.getElementById("videocard_total").value = 0;
 }
}