How to find a negative parabola equation from a graph

Question

I have a downwards facing parabola with:

X intercepts (0,0) and (6,0)

Vertex: (3,45)

How do I get the equation in standard form

Answer 1

y=a(x-3)^2 +45

0=9a+45

a=-5

y=-5(x-3)^2 +45 in vertex form

y=-5x^2 +30x in standard form