The perimeter is the distance all the way around a shape. This 3rd grade missing side length perimeter worksheet turns that idea around. Each shape already shows its total perimeter, but one side has a letter instead of a length. The first shape is a rectangle measured in feet, and the second one is a trapezoid with its sides measured in kilometres. The third is an L shape in millimetres, with six sides. So your child adds the sides they know and compares that sum with the perimeter. The difference is the missing side. Page two is the answer key, and it writes out every step, from the sum of the sides to the final answer.