Rectangular plot of land next to a river. 1,200 yards of fence make 3 pens with no fence next to river. find formula for y (longest side) and find formula for area of one pen. use diagram.

You are given with the total amount of fencing equal to 1,200 yards. This should be equated to the perimeter of all the fencing. The perimeter that needs fencing has the equation:

1,200 = y + 4x
Solving for y,
y = 1,200 - 4x   --> this is the equation for y

To find the area of one pen, assuming that all the pens have equal widths, the equation would be:

A = LW = (y/3)*x
A = xy/3   --> this is the equation for area

The variables should add up to equal 1