general form of the equation is
(x - a)^2 + (y-b)^2 = r^2
where (a,b)is the center and r = radius
so here we have
x^2 + y^2 = r^2
Now r^2 = square of distance between the points (0,0) and (5,2)
which = (5-0)^2 + (2-0)^2 = 25 + 4 = 29
so the required equation is x^2 + y^2 = 29