Triangle

Law of Cosines

6^2  = 7^2  + 5^2  - 2(7)(5)cosACB

[ 6^2 - 7^2  - 5^2 ] / [ -2 (7)(5) ] =  cos ACB

-38 / -70  = cos ACB  =  19/35

sin ACB =  sqrt [ 1 - (19/35)^2 ]  =  sqrt  [ 1 - 361/1225]  = sqrt [ 864]/ 35 = 12sqrt(6) / 35