writing equations

[porker39]

i dont know how to do this no matter how hard i try i dont understand

here is the question

write the standard form of the equation of the circle that passes through (0,5) and whose center is the orgin

 

[tkhunny]

Center is (h,k)
h = 0
k = 0
Radius is r
r = 5

x^2 + y^2 = 5^2