What is the geometric mean and the arithmetic mean of 7 x sqrt3 and 21 x sqrt3?

The arithmetic mean is simply (7+21)sqrt(3)/2 = 14sqrt(3) = 24.25 (to 2 d.p.) The geometric mean is sqrt [ 7sqrt(3) x 21sqrt(3) ] = sqrt [ 7 x 21 x 3 ] = sqrt [21 x 21] = 21