1. **State the problem:** Simplify the expression $90x + x$. 2. **Formula and rules:** When adding like terms, add their coefficients and keep the variable the same. 3. **Intermediate work:** $$90x + x = 90x + 1x = (90 + 1)x = 91x$$ 4. **Explanation:** Since both terms have the variable $x$, we add their coefficients $90$ and $1$ to get $91$, then multiply by $x$. **Final answer:** $91x$