The perimeter is the distance all the way around the edge of the shape.
It's like if you put an ant down on one corner of the shape, give it a kick
in the ant's pants and it starts walking along the line, and it keeps going
until it comes back to the corner where you put it down, how far would
the ant have to walk ?
Say we start from the lower-left corner, and we walk to the right, along
the bottom.
-- We walk 12 units to the right, and arrive at the lower right corner.
Then we turn and walk up.
-- We walk 2 units up, and arrive at the upper right corner.
Then we turn and walk left.
-- We walk 12 units left, and arrive at the upper left corner.
Then we turn and walk down.
-- We walk 2 units down, and arrive back where we started.
The total distance we walked around the outside is
12 + 2 + 12 + 2 = 28 units .
That's the perimeter of the shape.
Notice that we walked the length twice, and the width twice.
That leads us to the formula for the perimeter of any rectangle.
Perimeter of a rectangle = 2 (length + width) .
