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