A parabola has x-axis values of -3 and 5. And it symmetry axis is: x = 1

The a value is negative, but i cannot say what its exact value is.

It says to prove that $\displaystyle f(x) = -x^2 + 2x + 15 $

Here's what i did:

$\displaystyle f(x) = a(x + 3)(x - 5) $

$\displaystyle f(x) = a(x^2 - 2x - 15) $

And now i KNOW that $\displaystyle a $ must be equal to $\displaystyle -1 $ but i have no idea how to prove it

Please help guys.

EDIT: Sorry, i forgot to mention that there is also a graph $\displaystyle g(x) = |x| - 3 $