Question 866589
w=width
L=length
Formula for perimeter, p=2(w+L)
Given:  {{{L=w+5}}}
p=50


{{{2(w+L)=p}}}
{{{w+L=p/2}}}
{{{L=p/2-w}}}
Substitute,
{{{w+5=p/2-w}}}
{{{w+w=p/2-5}}}
{{{2w=p/2-5}}}
{{{w=(p/2-5)/2}}}
{{{highlight_green(w=p/4-5/2)}}}
Actual correct steps can vary some, but w, if in symbolic form must be equivalent to the green-outlined formula shown.  Just substitute for the given p value of 50, and compute w.  
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You could do all this without being in pure symbolic form - it's up to you; but it depends on how many of this general type of rectangle exercise you have.  This is a very frequent type of exercise problem and generalizing its solution can be more efficient if you have many of these.