1. **State the problem:** Simplify the expression $2a + 3a$. 2. **Formula and rules:** When adding like terms, add their coefficients and keep the variable part the same. 3. **Intermediate work:** $$2a + 3a = (2 + 3)a$$ 4. **Simplify:** $$= 5a$$ 5. **Explanation:** Since both terms have the variable $a$, we add the coefficients $2$ and $3$ to get $5$, then multiply by $a$. **Final answer:** $5a$