A = hw where width is the side with fencing

Cost (C) = (2h+w)48 + w(47)

Substitute in h = A/w

C = (2A/w + w)48 + 47w = 96A/w + 95w

Set dC/dw = 0 -96A/w^{2} + 95 = 0 , so w^{2} = (96/95)(5130) w = 72

I'll leave the rest to you. Note that 2nd derivative is 192A/w^{3} .> 0, so it is a minimum.