Start by collecting the x terms together, then the y terms:
Now complete the square in the x terms and the y terms:
Now you have to remember that the equation of a circle with centre (a,b) and radius r is . Compare that with the previous equation, and you see that this circle has centre (2,3) and radius .
Do the other two problems in the same way. For the next one, you may want to start by shifting the constant term across to the other side of the equation. For the last one, you'll want to do the same thing, plus divide through by 3 before completing the square.