Step-by-step explanation:
the perimeter is simply the distance you would have to walk when going around the whole object once.
so, you would go a length, a width, another length and another width, until you are finally arriving back at the starting point.
so, the perimeter of a rectangle is
2×length + 2×width = 2×21 × 2×11 = 42 + 22 = 64 m
