Equation of a line

<script>
function slopeCalc(){
  var x1=parseFloat(document.getElementById("x1").value);
  var x2=parseFloat(document.getElementById("x2").value);
  var x3=parseFloat(document.getElementById("x3").value);
  var y1=parseFloat(document.getElementById("y1").value);
  var y2=parseFloat(document.getElementById("y2").value);
  var y3=parseFloat(document.getElementById("y3").value);
  var b=(y1-x1*y2/x2)*x2/(x2-x1);
  var m=((y2-b)/x2);
  if(x3!=""){
      document.getElementById("y3").value=m*x3+b;
      return;
  }
  if(y3!=""){
      document.getElementById("x3").value=(y3-b)/m;
  }
}
</script>
<p>x1: <input type="text" id="x1" name="x1" value=""> y1:<input type="text" id="y1" name="y1" value=""></p>
<p>x2: <input type="text" id="x2" name="x2" value=""> y2:<input type="text" id="y2" name="y2" value=""></p>
<p>x3: <input type="text" id="x3" name="x3" value=""> y3:<input type="text" id="y3" name="y3" value=""></p>
<input type="button" name=" " value=" " onclick="slopeCalc();">