IF THEN tax calculator

I have an order form with a ComboBox with various Canadian provinces called "ComboBox2". Each province has a different tax rate. (For example, ON is 8%, QC is 7.5%, NS is 15%)

I then have a subtotal Field named "Subtotal".

The last field is a taxes calculator Field named "Taxes".

What I want is for the "Taxes" field to change the rate depending on what province was chosen from "ComboBox2".

This is the code I have so far, which seems logical, but is not working:

var a = this.getField("Subtotal");
var b = this.getField("ComboBox2");

if (b = "QC" ) event.value = Math.round (a.value * 15) / 100;
if (b = "ON" ) event.value = Math.round (a.value * 8) / 100;
if (b = "AB" ) event.value = Math.round (a.value * 5) / 100;

Any suggestions?

EDIT: Using Acrobat 8 Pro, not Standard as indicated below.