Have you drawn a picture? You actually have two right triangles, ACD and BCD.
And, because they are right triangles you don't need either the "sine rule" or "cosine rule".
Call the distance across the river "x". Then the total distance AC is 36+ x meters. That is the leg of the right triangle ACD opposite angle D. The "near side" has length 120 m. Solve the equation .
Call the angle that BC (NOT AB) subtends at D " ". Then it is an angle of right triangle BCD with "opposite" side of length 36 m and "near side" of length 120 m. . Once you have solved that for , the angle AB subtends at D is 56 degrees 20 minutes minus .