Within the scope of GMAT math, there is no way to simplify t^3 - m^3, so in order to answer the question, we'll need the values of both t and m.

Statement (1) is insufficient. We can factor and find that (t + m)(t - m) = 18, but that doesn't give us what we need.

Statement (2) is also insufficient. Two variables and one equation isn't enough to solve for the variables.

Taken together, the statements are sufficient. If t - m = 2, we can substitute that into the factored version of (1):
(t + m)(2) = 18
t + m = 9
Now we have two equations and two variables:
t + m = 9
t - m = 2
Add the equations:
2t = 11
t = 5.5
From there, we can find m and answer the question. No need to do the rest of the math, and the answer is (C).