1. **State the problem:** We are given a table of values for a function $f(x)$ and asked to evaluate $f(21)$ and find $x$ when $f(x) = 18$. 2. **Given table:** $$\begin{array}{c|cccccccccc} x & 0 & 3 & 6 & 9 & 12 & 15 & 18 & 21 & 24 & 27 \\ f(x) & 3 & 18 & 27 & 12 & 0 & 15 & 9 & 6 & 24 & 21 \end{array}$$ 3. **Evaluate $f(21)$:** From the table, when $x=21$, $f(21) = 6$. 4. **Determine $x$ when $f(x) = 18$:** Look for the value of $x$ where $f(x) = 18$ in the table. From the table, $f(3) = 18$, so $x=3$. **Final answers:** $$f(21) = 6$$ $$x = 3 \text{ when } f(x) = 18$$