The angle of elevation of the top of the tower is dependent on the height of the tower and how far away from the tower the person is standing.
When the person stands due south of the tower the angle of elevation is 26 degrees. Let the distance from the tower at which the person is standing be X m. We know that the height of the tower is 50 m. This gives: 50/X = tan 26
=> X = 50/tan 26 = 102.5 m
When the person walks a distance 120 m east, the distance from the tower is equal to sqrt(102.5^2 + 120^2) = 157.82 m.
The angle of elevation is arc tan(50/157.82) = 17.57 degrees.
The required angle of elevation of the top of the tower is 17.57 degrees.