g(x)= x+1 (fog)(x)= x²+3x+1 f(x) = ?

g(x) = x + 1
(fog)(x) = x² + 3x + 1
f(g(x)) = x² + 3x + 1
f(x + 1) = x² + 3x + 1
f(x + 1 - 1) = (x - 1)² + 3(x - 1) + 1
f(x) = x² - 2x + 1 + 3x - 3 + 1
f(x) = x² + x - 1