A campground owner has 1300m of fencing. He wants to enclose a rectangular field bordering a​ river, with no fencing along the river.​ (See the​ sketch.) Let x represent the width of the field. ​(a) Write an expression for the length of the field as a function of x. ​(b) Find the area of the field ​(arealength​width) as a function of x. ​(c) Find the value of x leading to the maximum area. ​(d) Find the maximum area.