Explanation:
In its raw form, function notation essentially represents an equation with only one unknown variable, expressed in terms of another. Thus, f(x) = x² + 7x can be expressed as
g(x) = 2x + 3
f(g(x)) = (2x + 3)²
f(g(x)) = 4x² + 12x + 9
Hope it helps :) and let me know if you want me to elaborate.