What is the equation of a circle with centre (3,4) and radius 4?

The general equation of a circle is (x-a)^2 + (y-b)^2 = r^2 where the centre is (a,b) and radius r.
Therefore the equation of this circle is (x-3)^2 +(y-4)^2 = 4^2 ...or 16
Sometimes its easiest to leave in r^2 form as expanding it can make work harder.