Question 1000584
Perimeter may match the 300 total feet retaining wall.  

w width, L length;
L=2w


{{{2L+2w=300}}}
and since the question is to find w, just substitute for L.
{{{2*2w+2w=300}}}
{{{2w+w=150}}}
{{{3w=150}}}
{{{highlight(w=50)}}}