a)
3m + 2m + 4m = (3 + 2 + 4)m = 9m
б)
\[\frac{1}{2}a + \frac{1}{3}a - \frac{1}{6}a = (\frac{1}{2} + \frac{1}{3} - \frac{1}{6})a = (\frac{3}{6} + \frac{2}{6} - \frac{1}{6})a = \frac{4}{6}a = \frac{2}{3}a\]
Ответ: а) 9m; б) \(\frac{2}{3}a\)