Question 871452
Let x = length
Let y = width
{{{2x+2y=300}}} according to perimeter description.
We want {{{x<y}}} because two lengths are used for 
fence but only one width, and we have a cost restriction.
Cost: {{{50*2x+50*y<=12000}}}.


The judgement of {{{x<y}}} is mostly a logical feeling, and not certain if it is really correct.  First just use the system:
{{{highlight_green(50*2x+50y<=12000)}}} and {{{highlight_green(2x+2y=300)}}} and see what happens.  Simplify each of those statements before using them.