calculations in a fillable form

i have a fillable form that i have created ttext fields in that i need to do multiple calculations to get a subtotal in the subtotal text filed i have created. I know how to do it if i am just addingor multiplying but in this case I need to multiple two text fields x 2 and then sum those totals together to get my subtotal like below:

Straight hours x hourly rate  +  OT Hours + OT hourly rate =Labour subtotal text field

please help i am thinking I need to do a custom  javascript but not sure how