Given the functions;
f(x) = 5x
g(x) = 2x-1
Required
The composite function f(g(x))
f(g(x)) = f(2x-1)
To get f(2x-1), we are to replace x with 2x-1 in f(x) as shown;
f(2x-1) = 5(2x-1)
Open the parenthesis;
f(2x-1) = 5(2x)-5(1)
f(2x-1) = 10x - 5
f(g(x)) = 10x - 5
Hence the composition f(g(x)) is 10x - 5