I really need help people!!!

I have three fields.

In the first, A Field, I type a price.

In the second, A ComboBox, I select a period of One month, six months or one year.

In the third, A Field, a price appears which is the price in the first field multiplied by the selection in the ComboBox.

I have tried everything but simply cannot seem to get the thing to work. Can someone possibly help me please!

Gazzer