[tex](g\circ f)(x)\text{ means put f(x) in g(x)}[/tex]
That means replace x in g by f
f(x) = -4x + 7
g(x) = 3x + 9
Substitute x in g(x) by f(x)
[tex](g\circ f)(x)=3(-4x+7)+9[/tex]
Now let us simplify it
[tex](g\circ f)(x)=3(-4x)+3(7)+9=-12x+21+9[/tex]
Let us add the like terms 21 and 9
[tex](g\circ f)(x)=-12x+(21+9)=-12x+30[/tex]
