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?