Which ordered pair is the solution to the system of equations? x+y=-1

3x-y=21

A. (5, - 6)

B. (7, - 8)

C. (5.5, - 6.5)

D. (10, - 11)

You use substitution and trial and error to answer this, so for A, every time you see an X you'd replace it with 5, and every time you see a Y you'd replace it with - 6. Luckily, the answer is a because after substitution, the sum is correct:

5 + (-6) = - 1

(3 * 5) - (-6) = 21