Show text field based on values of 2 other fields

Hi

I have some text fields that only need to be filled in if the values in 2 other fields meet certain conditions. One of these fields is a dropdown box the other is a tick box. So the hidden text field will appear if:

• The dropdown box = VALUE1

                         AND

• The tick box IS selected

Otherwise the textbox is hidden.

I'm not sure where to run the code as it is based on 2 conditions rather than 1.

Any help would be much appreciated.

Thanks