1/9 Here's the proper explanation: Since the vertex is (2,-1), the original equation will look like this: y=a*(x-2)^2-1 When the substitution happens we get to this: 0=a*(5-2)^2-1 then 1=a*(5-2)^2 -> 1=a*9 So, a=1/9! Written in another form: 1/9x^2-4/9x+4/9-1