Like terms have the same letter part, such as \(x\) terms with \(x\) terms or number terms with number terms. To simplify an expression, collect the like terms by adding or subtracting their coefficients. Keep the sign with each term and do not combine terms with different letter parts.
Common mistake: Only collect terms that are exactly alike. \(5m + 2n\) cannot be simplified to \(7mn\), because \(m\)-terms and \(n\)-terms are different.