Calculating Mass Using Standing Waves on a Suspended Wire

[physnoob]

Homework Statement

When mass M is tied to the bottom of a long, thin wire suspended from the ceiling, the wire's second-harmonic frequency is 200 Hz. Adding an additional 1kg to the hanging mass increases the second-harmonic frequency to 245 Hz. What is M?
f$$_{2}$$ = 200hz, m = M
f$$_{2}$$ = 245hz, m = M+1
g = 9.8m/s

Homework Equations

f=v/L, v = $$\sqrt{T/u}$$

The Attempt at a Solution

not sure where to start the problem
please help!
any help is greatly appreciated!

 

[physnoob]

Just did some plug and chuck with the equation f = v/L, substituted v = sqrt(T/u), T=mg, u = m/L, m cancels out, L = 2.45x10^-4, does this sound right?