In order to calculate the IRR they have used 11% (because that is the discount rate in the question) and 17% (which is a guess).
We always have to make guesses to calculate the IRR – if you are not sure what I mean than please do watch my free lectures on this (and if needed, my free Paper F2 lectures, because calculation of the IRR is revision from Paper F2).
(The lectures are a complete free course for Paper F9 and cover everything needed to be able to pass the exam well)