Use Einstein's equation.......
V=(hc/e)(1/$ - 1/@).......... Im using $ for incident wavelength and @ for cutoff wavelength
In case first:
V0=(hc/e) (1/$ - 1/@)............. (1)
In second case:
V0/6= (hc/e) (1/3$ - 1/@)........... (2)
Divide eq 1 by 2..... V0 and hc/e would cancel out.....
You'd get eqution with $ and @.......
Solving you'd get @=5$