基于买方类型的折扣计算

i'm learning JavaScript and am working on a JavaScript function to calculate discount based on type of buyer i.e retailer/wholesaler & if it's cash or check payment. Here is the function

<script>
    function sell()
    {

    var gross,net,tax,discount,unitprice,quatity,select2,select3;

    unitprice=parseInt(form9.unitprice.value);
    quantity=parseInt(form9.quantity.value);
    select3=parseInt(form9.select3.value);
    select2=parseInt(form9.select2.value);
    gross=parseInt(form9.gross.value);

    gross=unitprice*quantity
    form9.gross.value=gross;
    tax=0.16*gross
    form9.tax.value=tax;

        if(select2='Cash' && select3='Retailer')
         discount=0.07*gross;

        else if(select2='Cash' && select3='Wholesaler')
          discount=0.1*gross;

        else if(select2='Check')
          discount=0*gross; 
    form9.discount.value=discount;



    net=(gross-(discount+tax));
    form9.net.value=net;

    }
    </script>

I created a dropdown menu for type of buyer, named it select2

<select name="select2">
          <?php
do {  
?>
          <option value="<?php echo $row_rsCustomercategory['type']?>"><?php echo $row_rsCustomercategory['type']?></option>
          <?php
} while ($row_rsCustomercategory = mysql_fetch_assoc($rsCustomercategory));
  $rows = mysql_num_rows($rsCustomercategory);
  if($rows > 0) {
      mysql_data_seek($rsCustomercategory, 0);
      $row_rsCustomercategory = mysql_fetch_assoc($rsCustomercategory);
  }
?>
        </select>

The select menu for type of sale is select3

<select name="select3" id="select3">
        <?php
do {  
?>
        <option value="<?php echo $row_rsSaletype['type']?>"><?php echo $row_rsSaletype['type']?></option>
        <?php
} while ($row_rsSaletype = mysql_fetch_assoc($rsSaletype));
  $rows = mysql_num_rows($rsSaletype);
  if($rows > 0) {
      mysql_data_seek($rsSaletype, 0);
      $row_rsSaletype = mysql_fetch_assoc($rsSaletype);
  }
?>
      </select>

Cash sales are discount at 7% retailer and 10% wholesaler, no discount for payment by check.

The Problem is that: Upon viewing in a browser and clicking on the "Calculate" button nothing happens. Why is this? I would appreciate any help whatsoever.