The length of rubber cord is l1  meters when the tension is 4 N and l2  meters when the tension is 5 N. The length in meters when the tension is 9 N is

l1-l =kT1⇒l1 -l =k(4)----(1)l2 -l=k T2⇒l2-l =k(5)----(2)dividing (1) and (2) we get l=5l1 -4l2---(3)l3 -l = k(9)---(4)l2 -l =k(5)----(5)dividing (4) and (5) we get l=5l2 -4l1