Algebra: A Combined Approach (4th Edition)

$a^2 +2ah-3a+h^2-3h$
If $f(x) = x^2-3x$ $f(a+h) = (a+h)^2-3(a+h) = a^2+2ah+h^2-3a-3h = a^2 +2ah-3a+h^2-3h$