the formula for the equation of a circle is
\((x-h) ^{2} + (y-k) ^{2} =r ^{2}\)
The center (h,k) and the radius (r).
In this case the center is (2,4) so you would substitute -(h,k) with 2 and 4:
\((x-2) ^{2} +(y-4) ^{2} =r ^{2}\)
Then just substitute for r with 6:
\((x-2) ^{2} + (y-4) ^{2} =6 ^{2}\)
and there you go!