Find the equation of a circle. If a circle which center is on the straight line $x-2y+3=0$ cuts both the x-axis and the y-axis, what is the equation of the circle? 
ANSWER: $x^2+y^2-6x-6y+9=0$
This is rather a straightforward question but I don't know how to start...some hints would be very much appreciated.

Hint: Start with $\left(x-2a+3\right)^{2}+\left(x-a\right)^{2}=r^{2}$
