Write the perimeter down on a piece of paper. For an example, let the perimeter be 40 cm.
Count the number of sides of the shape. In this example, the shape is a pentagon with five sides.
Divide the perimeter by the number of sides. Concluding this example, dividing 40 cm by 5 sides results in 8 cm for each side.
Write down the perimeter. For this example, let the perimeter be 24 cm.
Write down the shape's perimeter formula and the relationship that the shape's sides have with each other. For this example, let the shape be a rectangle, which has a perimeter formula of 2*length + 2*width, and let the rectangle's length be twice as long as its width, or length = 2*width.
Substitute one of the measurements for another into the perimeter formula. In this example, 2*length + 2*width becomes 2*(2*width) + 2*width by substituting the length with 2*width.
Set the formula to the actual perimeter amount and solve. In this example, 2*(2*width)+2*width = 24 becomes 4*width + 2*width = 24 cm, which is also 6*width = 24. Solving for the width results in width = 4 cm.
Substitute the value of the known side into the side relationship from Step 2 and solve for the measurement of the unknown side. Concluding this example, substituting 4 into the relationship length = 2*width results in length = 2*4 cm and length = 8 cm.