The #2's after the letter s below represent squared. Not sure how to post it so it looks that way. Let me know if im doing this one right please.
9m2 -2mn + 5n2 - 9 (5m2 + 6mn - 8n2)
Step 1: Work whats in parenthesis (multiply)
-9 x 5m2 = -45m2 -9 x 6mn = -54mn -9 x 8n2 = -72n2
leaving: 9m2 - 2mn + 5n2 -45m2 - 54mn - 72n2
Step 2: combine like terms
9m2 + -45m2 = -36m2 -2mn + - 54mn = -56mn 5n2 + 72n2 = 77n2
Answer: 54m2 - 56mn +77n2
Comment on this one please....