Question 217213
Keep in mind that perimeter is 2*length plus 2*width.

{{{system( length = width+4, 2*length+2*width = 28 ) }}}

This is a linear system: 

{{{system( length -width = 4, 2*length+2*width = 28 ) }}}

which you can solve with the linear solver

*[invoke linear_substitution "length", "width", 1, -1, 4, 2, 2, 28 ]