A rectangular area adjacent to a river is fenced in; no fence is needed on the river side. The enclosed area is 2000 square feet. Fencing for the side parallel to the river is $ 7 per foot, and fencing for the other two sides is $ 4per foot. The four corner posts are $30 each. Let x be the length of one of the sides perpendicular to the river. The length of the side perpendicular to the river is how many feet?