You want to find the equation of this parabola. Scroll down:



(2, 4),(-2, 8),and (1, -4)



For the point (2, 4), substitute x = 2 and y = 4



For the point (-2, 8), substitute x = -2 and y = 8



For the point (1, -4), substitute x = 1 and y = -4



That gives us the system of three equations in three
unknowns a,b, and c



Simplify those equations:





Swap the sides of the equations so the system will look "normal":



Solve that by elimination.  Then substitute the values you get
for a, b, and c in 



If you run into any trouble finishing, ask me in the thank-you note 
form below, and I'll get back to you by email.  

Edwin