a) f(-1) = a(-1)² + 5(-1) - 6 = a - 11
f(1) = a(1)² + 5(1) - 6 = a - 1
so, f(-1) + f(1):
a - 11 + a - 1 = -8
2a = -8 + 12
a = 2
b) f(k) = k
a(k)² + 5(k) - 6 = k
2k² + 5k - 6 = k (a = 2, proved in (a))
2k² + 4k - 6 = 0
k² + 2k - 3 = 0
(k + 3) (k - 1) = 0
k = -3 or k = 1
