let x be equal to the width of the pen
let y be the length of the pen
and S be the area of a single pen
there are 6 pen to be fenced for 180 m;
6(2x + 2y) = 180 (eq. 1)
simplify;
6[2(x+y)]=180
12(x+y)=180
x + y = 15
then differentiate;
1 + y' = 0
y' = -1 (we will use it...