The rectangular area adjacent to the river is fenced in ; no fence is required on river side. Enclosed area is 2,000 square feet. Fencing for side parallel to river is $ 7 (dollars) per foot and fencing for other 2 sides is $ 4 (dollars) per foot. Four corner posts are 30 (dollars) each, consider x be length of sides perpendicular to river. Length of side perpendicular to river is how many feet?