Question 673665
the formula to find the perimeter of a rectangle is perimeter = 2(length)+2(width)

let width = w
the length is 3 times the width  therefore length = 3w

the perimeter is 84m

84=2(3w)+2w
84=6w+2w
84=8w
divide both sides by 8 to get w by itself
10.5=w

the length is 3w so 3(10.5)
answer: the length is 31.5 m

check answer:
84=2(3w)+2w
84=2(3x10.5)+2(10.5)
84=2(31.5)+21
84=63+21
84=84