Can You Explain How You Could Use A 10s Fact To Find A 9s Fact?

9 = 10 - 1

So, 9x = 10x - x

Example 1
If I know that 10 times 5 = 50, I can subtract 5 from 50 to find that 9 times 5 = 45.

Example 2
Suppose I want to learn 9*35. I can compute this as 10*35 - 35 = 350 - 35 = 315.

Sometimes this approach is much easier than multiplying by 9 directly.