Solve x² - 9x + 20 = 0 by factorisation.

Step 1. The pair. Product $20$, sum $-9$: $-4$ and $-5$.

Step 2. Factorise. $x^2 - 4x - 5x + 20 = x(x - 4) - 5(x - 4) = (x - 4)(x - 5)$.

Step 3. Solve. $x = 4$ or $x = 5$.

Step 4. Check. $16 - 36 + 20 = 0$ and $25 - 45 + 20 = 0$.