The formula to calculate the fundamental frequency produced in a vibrating string is given by
f0=2Lv...(1)
For the first case, it can be written that
120=2(90)v...(2)
Similarly, for the second case, it can be written that
180=2(L)v...(3)
Divide equation (3) by equation (2) and solve to obtain the required length of the string.
120180==⇒=2(90)v2(L)vL90L=90×18012060