How to call a method in a method

Hi,

I'm doing a calculation of withholding tax. I placed the withholding tax computation in a separate method like this:

display real disptax()
{
return CustTrans.AmountMST * 0.02;
}

Then I want to call this method in  a new method, calculating for

total amount = CustTrans.AmountMST  - (answer in disptax).

Please help me find a basic solutions for this 

Thank you.