1. **State the problem:** Evaluate the expression $x^2 + 5x$ for $x = 4$. 2. **Formula and rules:** The expression is a quadratic polynomial. To evaluate it, substitute the value of $x$ into the expression and simplify. 3. **Substitute $x = 4$:** $$4^2 + 5 \times 4$$ 4. **Calculate powers and products:** $$16 + 20$$ 5. **Add the results:** $$36$$ **Final answer:** The value of $x^2 + 5x$ when $x = 4$ is $36$.