Rectangle Perimeter

Two lengths and two widths make the trip around, so $P = 2(l + w)$. It is the baseboard formula, the fence formula, the picture-frame formula: a 4 m × 3 m room needs 14 m of trim, minus the doorways. Solving backwards is just as common — with 30 ft of edging and a 10 ft side already fixed, the other side must be 5 ft.

The contrast with area is worth holding onto. Perimeter grows in step with size while area grows with its square, so of all rectangles with a 20 m perimeter, the 5 × 5 square encloses 25 m² while a long thin 9 × 1 encloses only 9 m². Same fencing, very different paddock.