Answer:
1 = 21 m
2 = 120 cm
3 = 84 cm
4 = 3 km
5 = 10 m
6 = 55 mm
7 = 60 cm
8 = 500 mm
Step-by-step explanation: you see where it says "perimeter = 3s" right? for each question you have to replace "s" with the number given so for the first one it would be 3 x 7. You had to do the same thing with the others but sometimes we get confused in math :D