A pole, base at O, is on the west bank of a river. Two points, A and B, stand on the east bank of the river whose banks run parallel north to south. A to the south of O and B to the north. the distance AB is [tex]2a\sqrt{7}[/math and the angle AOB is . If the angle of elevation of the top of the pole, P, from A is and from B, find in terms of a the height of the pole and the width of the river.

I know OP=OA and OA=

and but i don't know how to eliminate the sinB

thanks