You wanna write an equation to find out what the two lengths will be. Because the string in 17 feet the equation has to equal 17. Then for the shorter length you need to symbolize it with a variable (x). For the longer side it is the smaller number plus five(x+5).
Therefore, the equation is x+(x+5)=17
from there you get x= 6ft which leaves the longer side equaling 11ft
