For the first one, estimate. It's rougly 3x5x10a + b or 1.5x 10a+b+1, so the answer is C.
For the second, notice that every four times the nubmer at the end of the power of three repeats in order 3, 9, 7, 1. What you should notice is that the last digit is 3 if the power raised by is n=1 (mod 4), and it happens 215 = 3 (mod 4), so the answer is 7, D.
To explain what I just did, n=1 (mod 4) means that "when n is divided by 4, the remainder is 1" (The = should have 3 lines rather than two, since it's not possible to solve for n - this is a congruence, not an equation). So you divide 215 by 4 and find that there is a remainder of 3. (If you need help with this, PM me and I'll try to explain further.)