Like it is stated, combine like terms. In case you don't know what that means, then the question is essentially asking for you to add/subtract etc. expressions from each other that have the same variable.
[tex]Given: -\frac{2}{3} a + \frac{5}{6}a - \frac{1}{6} \\\\[/tex]
[tex]Note: -\frac{2}{3}a = -\frac{4}{6}a[/tex]
[tex]-\frac{4}{6}a + \frac{5}{6}a - \frac{1}{6} = \\\\\frac{1}{6}a - \frac{1}{6} \\[/tex]
So, [tex]\frac{1}{6}a - \frac{1}{6}[/tex] is the new and equivalent expression. We cannot do anything further because there are no like terms.
