The equation is x^2+y^2=25. To derive this we draw a circle of radius 5 with its centre at the origin of the x-y axes. Let a point P(x,y) be on the circumference of the circle. If we join the point to the origin by a straight line, we will in fact draw a radius, which has a fixed length of 5. It's also the hypotenuse of a right-angled triangle with the other sides length x and y. By Pythagoras x^2+y^2=5^2, and this is true for every point on the circumference so x^+y^2=25 is the equation of the circle.