The Width Of A Rectangle Is Fixed At 26cm. What Lengths Will Make The Perimeter Greater Than 80cm?

The perimeter of a rectangle is given by
perimeter = 2(length + width)
You want
perimeter > 80 cm
We can substitute the known quantities and relations ...
2(length + 26 cm) > 80 cm
length + 26 cm > 40 cm (divide both sides by 2)
length > 14 cm (subtract 26 cm from both sides)

Lengths greater than 14 cm will make the perimeter greater than 80 cm.