Hello Andre,

Use "\not" as you have, bust surround the expression you want to cross out with curly brackets: {...}. Also, to avoid confusion of multiplication "dots" with decimal "dots", use "\cdot" which gives a dot but not at the line base:

$$\frac{20! 6!}{18! 5!} = \frac{20\cdot19\cdot\not{18!}\cdot 6 \cdot \not{5!}}{18! 5!} = 20 \cdot 19 \cdot 6 = 2280$$