The length of a metal wire is l1 when the tension in it is F1 and l2 when the tension is F2. Then original length of the wire is:
F∝Δl
F1∝l1−l
Where l, is the original length of the wire
F2∝l2−l
F1F2=l1−ll2−l
F1l2−F1l=F2l1−F2l
(F2−F1)l=F2l1−F1l2
l=F2l1−F1l2F2−F1