HELP MAX POINTS

So previously I answered,

(a) Start by writing the equation of the circle. (Recall that the general form of a circle with the center at the origin is x2 + y2 = r2. (10 points) x^2 + y^2 = 30^2

and

(b) Now solve this equation for y. Remember the roller coaster is above ground, so you are only interested in the positive root. (10 points) y = √(900 - x^2)


Now I have to solve this in the image attached,

Any ideas?