Question 1068091
y - 30 = a(x-5)(x-15) satisfies the first two conditions. Next we use the third constraint, which is

when x=20 we have 0 - 30 = a (20-5)(20-15) = 75a

or a = -30/75 = -10/25 = -0.4

I get y = -0.4 x^2 + 8x + 30

Let me know if this helps