Need help with zeroing calculation variables

I have created a series of 3 slides that use variables to perform calculations.  On the first slide the user enters the # of credits he/she is planning to take.  The second variable multiples the # of credits by 2 and gives the total in a 3rd variable when the calculate button is pressed.

I have all of that working great, but the problem I'm running into is if someone changes the # of credits and then hits calculate again it is adding it all together and making a mess.

Does anyone know what I need to do to get it to zero out each time and recalculate?

Thanks,

Stephanie