So the width of the rectangle is x. You would write an equation as x+(3x+4)=56.
You add because you are finding the perimeter. You would simplify the equation down to 4x+4=56. You take the four out by subtracting it on both sides, the four itself and 56. You would be left with 4x=52. You then divide 4x by 4 to cancel out the 4 to have just x and also divide 52 by 4, and you would get 13. Then you substitute all x by 13. Your equation would be 13+(3(13)+4)=56. You then simplify and get 13+43=56. You then add and get 56=56.