We can use the Law of Cosines
AC^2 = AB^2 + BC^2 - 2(AB* BC) cos (ABC)
40^2 = 25^2 + 25^2 - 2( 25*25) cos (ABC)
cos ABC = [ ( 40^2 - 25^2 - 25^2 ) / ( - 2 * 25 *25) ]
cos ABC = - 7/25
sin ABC = sqrt ( 1 -(-7/25)^2 ) = sqrt [ 625 - 49] / 25 = sqrt [ 576] / 25 = 24 / 25