Hey Razbaz.
The difference of two cubes formula is x^3 - y^3 = (x-y)(x^2 + xy + y^2). (Hint let x = a - 2 and guess what y should be).
Basically, trying to factorize (a-2)^3 - (a+1)^3, the workbook unit's about using the sum and difference of 2 cubes.
Any help would be appreciated
Btw, the answer was -9(a^2 - a + 1)