Vertex form of parabola is
y = a(x - b)^2 + c where a is a constant and the vertex is at the point (b,c)
so here we have
y = a(x - 4)^2 + 7
To find value of a we plug in the point (3,6)
6 = a(3 - 4)^2 + 7
6 = a + 7
a = 6-7 = -1
so equation is y = -(x - 4)^2 + 7
Covert to standard form:-
y = -(x^2 - 8x + 16) + 7
y = -x^2 + 8x - 9