Find sin \angle ACB.
Law of Cosines
4^2 = 7^2 + 5^2 - 2 ( 7 * 5) cos ACB
[ 4^2 - 7^2 - 5^2 ] / [ - 2 * (7 * 6) ] =cos ACB
29 / 42 = cos ACB
sin ACB = sqrt [ 42^2 - 29^2 ] / 42 = sqrt [ 923 ] / 42
