The side of a triangle with 3 equal sides is 7 inches shorter than the side of a square. the perimeter of the square is 39 inches more than the perimeter of the triangle. find the length of a side of the square.

The first thing we must do for this case is to define variables.
 We have then:
 x: sides of the triangle
 y: sides of the square
 The perimeter of the triangle is:
 
 The perimeter of the square is:
 
 We now write the system of equations that models the problem.
 The side of a triangle with 3 equal sides is 7 inches shorter than the side of a square:
 
 The perimeter of the square is 39 inches more than the perimeter of the triangle:
 
 Resolving the system graphically we have that the solution is the ordered pair:
 
 Note: See attached image for graphic solution.
 
 The length of a side of the square is: