cubic means 3
having a zero of n is the equivalent of
x-n = 0
thus having zeros of -4,-5, and 4 is the equivalent of
x+4=0
x+5=0
x-4=0
hence
(x+4)(x+5)(x-4)=0 must be valid
now, finding the zero of an equation involves setting the y- coordinate point as 0, so working backwards
y=(x+4)(x+5)(x-4)
just expand the equation to get to standard form
