Find the equation whose graph is a parabola with vertex (2,4), vertical axis of symmetry, and contains the point (1,-1). Express your answer in the form "ax^2+bx+c".
-b / 2a = 2
b = -4a
ax^2 - 4ax + c.
a - 4a + c = -1
-3a + c = -1
plug in 2
4a - 8a + c = 4
-4a + c = 4
Subtract equation 2 from equation 1
a = -5
c = -16
b = 20
-5x^2 + 20x - 16
