Explanation:
but that is the easiest of all 4 questions (by far) :
3g(x) + h(x) = 3×(5x² + 3) + 9x² = 15x² + 9 + 9x² = 24x² + 9
which is exactly f(x) = 24x² + 9
so, yes, they are equal.
I really have to wonder, since you correctly solved 1-3, what was the issue with 4. ?