The equation for a circle is (x-h)^2+(y-k)^2 = r^2
The h part in with the x is your x coordinate and the k part in with the y is your y coordinate. The r stands for the radius but be careful you have to square it.
so:
(x-(4))^2 +(y-(6)^2 = 16
I like to add the parenthesis around the "plugged in" numbers to help me remember negatives if I have any.