The definition of the tangent function tells you
tan(angle) = (300 ft) / (distance to mountain)
This equation can be rearranged to
(distance to mountain) = (300 ft) / tan(angle)
For the far end of the river,
distance to far end = (300 ft) / tan(24°) ≈ 673.8 ft
For the near end of the river
distance to near end = (300 ft) / tan(40°) ≈ 357.5 ft
Then the width of the river can be calculated by finding the difference of these distances:
width of river = distance to far end - distance to near end
width of river = 673.8 ft - 357.5 ft
width of river = 316.3 ft
The appropriate answer choice is
316 ft.