my teacher for our first homework assignment gave us half answers, but i cant get the correct answer for this:

$\displaystyle (x^2 - 14x - 51)/(x^2 - 4x - 21)$ when you plug in -3 is equal to what?

my math teacher says 2 is the answer, but i always get $\displaystyle (9 + 42 - 51) / (9+12 -21) = (51-51) / (21-21) = 0/0$

is my math teacher wrong? or what am i doing incorrectly?