perimeter is the sum of all a shapes sides.
with that in mind, we know a rectangle has four sides. with 2 pairs being the same.
The length as told is, 5 centimeters less than two times the width. Two times the width would be 2w and five less than that would be minus five. so the length is 2w - 5
now to find the perimeter we do two times the width + two times the length.
2w + 2(2w -5) = 80.
Distribute: 2w + 4w - 10 = 80
Add ten to both sides and combine the 2w & 4w:
6w = 90
Divide both sides by 6: w = 15.
with this we know the width is 15. to find length plug in 15 to the 2w - 5 equation.
2(15) - 5 = 30 - 5 = 25.
Slap on the units and width = 15 cm, length = 25 cm