Hello again, all. This comes from my Calculus summer assignment, but it has to do with a geometric formula, so I thought it would make sense to post it here. I don't know how to start, because I lack a center.

Find the equation of the circle that passes through the origin and has intercepts equal to 1 and 2 on the x- and y- axes, respectively.

No idea how to start. The equation format of a circle is: (x-h)^2 + (y-k)^2 = r^2, where the center of the circle is (h,k) and the r is the radius of the circle.