Question 1155358
Draw the figure and if choosing x for side perpendicular to river and y as side opposite the river, then:


{{{2x+y=2400}}}
{{{y=2400-2x}}}
and if A is AREA, then  {{{A=xy}}}
{{{highlight_green(A=x(2400-2x))}}}


A is a parabola with a maximum vertex point.
Zeros are x at 0 and at 1200.
Maximum A occurs for {{{highlight(x=600)}}}.


Other dimension is  {{{y=2400-2*600=2400-1200=highlight(1200)}}}.


Maximum area is  720000 square feet.