Why is it when it says Sin(A) he puts theta which is the c angle? and when it says Sin(B) the A angle is put? is there something im missing 
+128406 Remember that cos (120) = -cos (60) = -1/2
So....using the Law of Cosines
BC^2 = x^2 + (3x)^2 - 2 (x) (3x) (-1/2)
BC^2 = x^2 + 9x^2 + 3x^2
BC^2 = 13x^2
BC = x sqrt (13)
So.....using the Law of Sines
Sin (BCA) / x = sin (120) / [ x sqrt (13) ]
Sin (BCA) = sqrt (3) / [ 2sqrt (13) ] =
Sin (BCA) = sqrt (39) / [ 2 * 13 ] = sqrt (39) / 26
