you need to solve this system for a and y:-
x^2 + y^2 = 2025
y = (1/20)x^2 + 15 
substituting:-
x^2 + ( 0.05x^2 + 15)^2  = 2025
x^2 + 0.0025x^4 + 1.5x^2 + 225 = 2025
0.0025x^4 + 2.5x^2 - 1800 = 0
solving this  gives x^2 = 484.88
so x =  +/- sqrt 484.88 
find the y coordidnates by substituting in the first equation