Composing functions means to use the output of the inner function as the input for the outer one. It gets really clear and simple if you explicitly write it out:
[tex](f\circ g)(x)=f(g(x))[/tex]
Now substitute [tex]g(x)=9x[/tex]:
[tex](f\circ g)(x)=f(g(x))=f(9x)[/tex]
Now, since [tex]f(x)=2x[/tex], we know that [tex]f(x)[/tex] gives as output twice the input. In this case, the input is [tex]g(x)=9x[/tex], so the output will be twice as much:
[tex](f\circ g)(x)=f(g(x))=f(9x)=18x[/tex]
Note that we also have
[tex](g\circ f)(x)=g(f(x))=g(2x)=9\cdot 2x=18x[/tex]
