1. **State the problem:** We need to find an expression for "a certain number decreased by 5 twice the number." Let the certain number be $x$. 2. **Translate the problem into an algebraic expression:** "Twice the number" means $2x$. 3. **Decreased by 5 twice the number** means subtracting $5 \times 2x$ from the number $x$. 4. **Write the expression:** $$x - 5 \times 2x$$ 5. **Simplify the expression:** $$x - 10x = -9x$$ 6. **Interpretation:** The expression simplifies to $-9x$, which means the result is negative nine times the original number. **Final answer:** $$-9x$$