Find sin \angle ABC.
Law of Cosines
5^2 = 4^2 + 7^2 - 2(4 * 7) cos (ABC)
[5^2 - 4^2 - 7^2 ] / [-2 (4 * 7] = cos ABC
-40 / -56 = cos ABC = 5 / 8
sin ABC = sqrt [ 8^2 - 5^2 ] / 8 = sqrt [39 ] / 8
