Simplify and justify steps 4 (a+2b) = 3 (2a-b) + 6a-7b

4 (a + 2b) = 4a + 4 * (2b) = 4a + 8b;

3 (2a - b) = 3 * (2a) - 3b = 6a - 3b;

3 (2a - b) + 6a - 7b = 6a - 3b + 6a - 7b = 6a + 6a - 3b - 7b = 12a - 10b;

Finally, 4a + 8b = 12a - 10b;

8b + 10b = 12a - 4a;

18b = 8a;

9b = 4a.