1. The problem is to simplify the expression $5d + 2f$. 2. This expression is a sum of two terms involving different variables: $d$ and $f$. 3. Since $d$ and $f$ are different variables, the terms cannot be combined further. 4. Therefore, the simplified form of the expression is the same as the original: $5d + 2f$. 5. This is an example of a linear expression with two terms, each multiplied by a coefficient. 6. No further simplification is possible unless values for $d$ or $f$ are given.