Tuesday's (Too Easy) Teaser

Make 'em harder they kept saying.  Alright, starting next week. In keeping with our literal equation theme of late, today's teaser requires those same skills. Email showing the equation solved for the proper variable in addition to the correct answer before Friday to get the 10 extra credit points. 

 Next week, promise.

  Find the length of a field that is 50 yards wide and has a perimeter of 220 yards.