| page 427, #15.
We know that x = -3 at point P and x = 3 at Q (b/c PQ = 6, and can be divided into 3 on each side b/c parabolas are symmetric, and y = x^2 specifically is symmetric about the y-axis).
Plug in to y = x^2 to find the y value. y = 3^2 = 9.
Now plug into the other equation, with y = 9 and x = 3...
9 = a - (3)^2
9 = a - 9
18 = a |