Question 10695
Let X be the width of the rectangle in inches:   W = X.
The length of the rectangle is 3 times the width : L = 3X.
The perimeter of the rectangle : P = 2 W + 2 L
                                 P = 2 ( W + L )
Replace W and L by X :           P = 2 ( X + 3X )
Replace P by 56 "    :          56 = 2(4X)
                                56 = 8 X
                                 X = 56/8
                                 X = 7

The Dimention of the rectangle Width 7", Length 21"