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

Here's what i did:

And now i KNOW that must be equal to but i have no idea how to prove it

Please help guys.

EDIT: Sorry, i forgot to mention that there is also a graph