Think of this question as exploring the ideas we looked at in 5.1. For part (a) of 5.21, we want to make tables of the multiples of 5 and 6 (mod 8) like we did in class for multiples of 3 (mod 11):

For part (b), we want to find mod. mult. inverses for numbers (mod) 8 (where they exist). Recall they only exist for numbers relatively prime to 8. For example, 1 is it's own inverse, as is 7. What about the other number relatively prime to 8?

Hope this helps! Let us know if you have any follow-up questions.