Here you have a composite function, f(g(x)) involving the two functions f and g. You can think of x as an input - you first input x into the function g, and then take the result of that and that becomes the "x" input for f.
The first step of plugging x into g, we are given that the result is g(x)= 2x^3 + 10x-7. So there is nothing more to do as far as g(x) is concerned....Now that expression becomes the "x" input to f.
Since f(x) = 4x + 5, we substitute 2x^3 + 10x-7 for x in f(x) -->
f(g(x)) = f(2x^3 + 10x-7)
= 4(2x^3 + 10x-7) + 5
= 4* 2x^3 + 4*10x - 4* 7 + 5
= 8x^3 + 40x -28 + 5
= 8x^3 + 40x -23
