1. **State the problem:** Simplify or evaluate the expression $2a + 5b - 4$ if values for $a$ and $b$ are given, or understand its structure as an algebraic expression. 2. **Understand the expression:** This is a linear algebraic expression with two variables $a$ and $b$, and a constant term $-4$. 3. **No further simplification is possible** unless values for $a$ and $b$ are provided. 4. **If values are given, substitute and simplify:** For example, if $a=1$ and $b=2$, then $$2(1) + 5(2) - 4 = 2 + 10 - 4 = 8.$$