C
given the 2 equations
8(a + b) + 3z = - 9 → (1)
(a + b) + z = 2 → (2)
multiply (2) by 3 so that the coefficients of z are equal
3(a + b) + 3z = 6 → (3)
subtract (1) - (3) to eliminate term in z
5(a + b) = - 15 ( divide both sides by 5 )
(a + b) = - 3 → C
