Since \(ba=ab\), the first two terms combine as \(3ab+4ab=7ab\).

The last term contains no factor \(b\), so it cannot be collected with \(7ab\). For example, \(a=1,b=2\) gives \(12\), while \(5ab\) gives \(10\).

Answer

\(3ab+4ba-2a=7ab-2a\). The terms \(7ab\) and \(-2a\) are not like terms.