If g(x) = x^2+2x, find g(a - 1)

g(x) = X^2 + 2X.

g(a-1) = (a-1)^2 + 2(a-1),
g(a-1) = (a-1)((a-1) + 2),
g(a-1) = (a-1)(a + 1).