The equation for a circle with center (h,k) and radius r is:
So you know r = 3. Where's the center?
Recall that it lies on the two lines given, so find the intersection of the two lines. I did it by substitution, starting with the first equation, which gives:
Sub this into the other equation for y, which gives:
Recall , so .
So your center is at . Now you know h, k, and r, the three essentials for writing the equation of a circle.
There you are!