the first step is to draw a diagram. i guess we can skip that here

the next step is to find your objective and constraint equations.

(i suppose you want a rectangular fence?) if so, let the length be x and the width be y.

since we have 120m of fencing, we have that 2x + 2y = 120 => x + y = 60 ....this is your constraint equation.

the area of the fence is given by

A = xy

this is your objective equation.

do you know what to do from here?