Here in this question to find the coefficient of X raise to power minus 13 we need to get x² from (1+x)(2-x)¹⁷ so we have to make cases and we can make two cases where first we take x² from the equation (1+x)¹⁷ and x⁰from (1-x) and the second case will take x¹ from both the brackets and thus if we add the coffecicients of x^-13 from both cases we get ¹⁷C2 -¹⁷C1 but in book it's just ¹⁷C2 [enter image description here][1]


 [1]: https://i.sstatic.net/AJ5gpbe8.jpg