$\displaystyle f(x) = \frac{x + 3}{2}$

Given that $\displaystyle f(-9) + f(t) = A + Bt$, find the values of A and B.

Please help me in solving this problem. It's a nightmare. (Headbang)

For reference the answers are,

A is $\displaystyle -\frac{3}{2}$

B is $\displaystyle \frac{1}{2}$