Lets assume that this resonances appear as n and p harmonic in the string where n>p
We know that a fixed string like this have the wave length `lambda` as following depending on the harmonics.
`lambda` = 2/n(L) where L is the length of the string and n is the number of harmony.
So for n th harmony velocity = 420*2/n(L)
For p th harmony velocity = 490*2/p(L)
Since both locations we have same velocity in string;
420*2/n(L) = 490*2/p(L)
p/n = 490/420=7/6
So n=6 and p=7
In fundamental frequency;
lambda = 2L
So velocity = `f_0` *2L
By comparing with one of the harmonics previously found we get;
`f_0` *2L = 490*2/p*L
`f_0` = 70 Hz
So the fundamental frequency is 70 Hz.