$l_1-l =k{T}_1\Rightarrow l_1 -l =k\left(4\right)----\left(1\right)$

$l_2 -l=k T_2\Rightarrow l_2-l =k\left(5\right)----\left(2\right)$

$dividing \left(1\right) and \left(2\right) we get$

$l=5l_1 -4l_2---\left(3\right)$

$l_3 -l = k\left(9\right)---\left(4\right)$

$l_2 -l =k\left(5\right)----\left(5\right)$

$dividing \left(4\right) and \left(5\right) we get$

$l=5l_2 -4l_1$