The ideal gas law is PV=nRT. We can use a simplification of it in this case:
PV/T = nR, so since nR will stay the same we can say PV/T = PV/T (the surface values on the left side, and the ocean depth values on the right side). This version is called the "combined gas law" in some chem classes (as compared with Boyle's Law and Charles' Law).
So just like familiar ratio problems with two variables, you plug in values for all but one of them and solve for the last one. Five of the six values are given, jut plug them in.