How do I make a form with a formula so that two numbers from other forms can be multiplied and then

Use this as custom calculation script of "Field4":

var f1 = Number(this.getField("Field1").valueAsString);
var f2 = Number(this.getField("Field2").valueAsString);
var f3 = Number(this.getField("Field3").valueAsString);

if(f3 != 0)
 event.value = f1*f2/f3;
else
 event.value = 0;

Replace "Field1", "Field2", "Field3" with your actual field names.