let h is the hight of the lighthouse. we have tan15.7 = h/3000 -->>> h=3000tan15.7 and since
tan15.7 = 0.28108 we have h=3000 x 0.28108 = 843.26 m (very high building I must say)
now let e is the elevation angle. we have tan(e)=h/5000 --->>> tan(e) = 843.26/5000 = 0.16865
so the elevation angle is the angle having tan value = 0.16865