Chapter 25 Further Up and Further In
Solving higher-degree polynomial congruences, like x3≡a (mod n). (Chapter 7)
Knowing how to find the first nontrivial integer point on hard things like the Pell (hyperbola) equation x2−ny2=1. (Chapter 15)
Writing a number not just in terms of a sum of squares, but a sum of cubes, or a sum like x2+7y2. (Chapter 14)
The Prime Number Theorem, and finding ever better approximations to π(x). (Chapter 21)