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Let the width   = W

Let the length =  W + 3

The perimeter  =  2 ( W + L)

So

58 =  2 ( W  + (W + 3) )      divide both sides by  2

29  = 2W + 3         subtract  3  from both sides

26  = 2W     divide both sides  by  2

13 = W  = the width

And the length  = W + 3   = 16

The length of a rectangle is 3 more than its width. The perimeter of the rectangle is 58 cm. What are the rectangle’s dimensions?

( 58 - 2*3) / 4 = 13            Rectanle sides are:  a = 13cm   b = 16cm