Question 896678
Yes you're doing right.

{{{f(u) = 3u^2 + 3u -4}}}

{{{g(x) = x+1}}}

{{{f(g(x)) = y = 3(x+1)^2 + 3(x+1) - 4}}}

{{{y = 3x^2 +6x + 3 +3x +3 -4}}}

{{{y = 3x^2 + 9x + 2}}}