The EASIEST way to do it, is actually start with the form a(x-b)(x-c) or intercept form since you have intercepts
x ints at (0,0) and (4,0) so y=ax(x-4)
sub in (2,-4) it doesn't really matter whether or not its a vertex
-4=2a(2-4)
a=1
so y=x(x-4)
expand to get y=x^2-4x which is y=ax^2+bx+c form where a=1, b=-4, c=0
