The length of an elastic string is a meter when the longitudinal tension is 4 N and b meter when the longitudinal tension is 5 N. The length of the string in meter when the longitudinal tension is 9 N is

Using Hooke’s law, F=Kx we can write 4=k(a−ℓ0) …. (i) and 5=k(b−ℓ0) …. (ii)Ifℓ be the length under tension 9N, then9=k(ℓ−ℓ0) …. (iii)After solving above equations, we get ℓ=(5b−4a)