Use Einstein's equation.......

V=(hc/e)(1/$ - 1/@).......... Im using $ for incident wavelength and @ for cutoff wavelength

In case first:

V_{0}=(hc/e) (1/$ - 1/@)............. (1)

In second case:

V_{0}/6= (hc/e) (1/3$ - 1/@)........... (2)

Divide eq 1 by 2..... V_{0} and hc/e would cancel out.....

You'd get eqution with $ and @.......

Solving you'd get **@=5$**