Say the width is W and the length is L
L=3W (and of course W=W)
Now you use the perimeter formula P=2L+2W
80=2(3W) + 2W)
80=6W+2W
80=8W Divide both sides by 8
W=10
Now that you know the width, substitute it back into L=3W to find the length
L=3(10)
L=30
The dimensions are length: 30 cm and width: 10 cm