I understand how to solve recurrence relations, however when given a problem like the one below I'm not sure what I need to do.

I've searched google but all I ever find is how to solve the recurrence relation not how to compute like the problem below. Can someone please

explain it too me? Thank you.

Compute T(n) for n = 2^k

T(n) = a for n ≤ 2

T(n) = 8T(n/2) + bn^2 for n > 2