A farmer plans to use $120$ meters of fencing

to build a rectangular area to grow date trees

with the side of a building serving as one of

the longer sides. If $x$ represents the width of

the area and the techniques of differential

calculus are used, solving which of the

following equations is a step in finding the

maximum possible area to plant his date

trees?

$x^2-120=780$

$900-x^2=0$

$120-4x=0$

$9x-120=150$