Yeah, once you've figured out which sides of the triangle you are dealing with, it dictates which of the trig ratios you are using. It all comes back to SOHCAHTOA.

If you have the opposite and adjacent you will always use tan.
If you have the adjacent and hypotenuse you will always use cos.
If you have the opposite and hypotenuse you will always use sin.

That's why it's important to label the sides of the triangle that are important for the question (labelling that third side is a bit redundant, you only want to look for two of O, A and H).

Find the angle (not the right angle) you are using and figure out which sides are which, set things up properly and you'll never go wrong.

Remember that SOHCAHTOA not only tells you which sides go with which trig ratio, but which one's on top of the fraction as well.

Sam's upset, gotta go... hope this makes sense

BW