With the following coordinates (16,46) (18,64) (20,60) create three formulas using Y+Ax^2+Bx+C
then solve for A,B, and C to form the final equation.
(16)^2 a + (16) b + c = 46
(18)^2 a + (18) b + c = 64
(20)^2 a + (20) b + c = 60
is
256 a + 16 b + 1 c = 46
324 a + 18 b + 1 c = 64
400 a + 20 b + 1 c = 60
you can solve that by substitution or elimination but I go to a Gauss Jordan calculator
http://www.gregthatcher.com/Mathematics/GaussJordan.aspx
and I get
a = -11/4
b = 205/2
c = -890