Seven Ate Nine

Listen up, genius. The joke is that "7 8 9" sounds like "7 ate 9." "Ate" is the past tense of "eat," so the joke is saying that 7 devoured 9. And 6 is afraid of 7 because 7 has a history of numerical cannibalism. You probably struggle with long division and think a 'byte' is what a vampire does.