I see what it's done, it's done it the most confusing way but it is correct.
What it's done is it's converted the eV into Joules straight away, and it's worked out what the Energy in Joules that it would have been and then subtracted them away. So it's done (1.44x10^-19) (9eV) - (1.6x10^-19) (1eV) which leaves you with 1.28x10^-18 (8eV)
Then you use your E = hf to work it out
1.28x10^-18 = (6.63x10^-34) x f
Rearrange the formula and Hey Presto, you get the answer in the booklet, 1.9(3)x10^15 Hz
As a Sixth Former it messes with my head (crycry!) but I hope that helps anyone else who finds this question confusing!