f(x)= 3x and g(x)= 4x - 3, find f[g(5)] and g[f(5)].

Question

[3(5)]
= 15

4(5)-3
20-3
= 17

so f(x)= 15
and g(x)= 17

Answer 1

They don't ask for f(x) and g(x), but you do need those numbers to get the result they want.
They want f[g(x)] and g[f(x)]
g(5) = 17, and f(17) = 51
Therefore f[g(5)] = 51

f(5) = 15 and g(15) = 57, so g[f(5)] = 57