They gave you x and x+5 and the area is given which is 36, the opposite sides of a rectangle is equal so you assume the opposite sides are equal and add them together x+5+x+5= 10+2x and x+x= 2x then add 10+2x+2x=36 which you simplify to 10+4x=36 , subtract 10 from both sides which leads you to get 4x=26 then do 26/4 which leads you to get x=6.5
