1. The problem asks how to substitute domain values into a cost function from a previous problem. 2. Generally, a cost function is written as $C(x)$ where $x$ is the domain variable (e.g., quantity, time). 3. To substitute a domain value, replace $x$ in the function with the given number. 4. For example, if $C(x) = 5x + 10$ and the domain value is $x=3$, substitute to get: $$C(3) = 5(3) + 10$$ 5. Simplify the expression: $$C(3) = 15 + 10 = 25$$ 6. This means the cost when $x=3$ is 25. 7. Always ensure to substitute the exact domain value into every instance of $x$ in the function. 8. If the function is more complex, follow the same substitution and simplify step-by-step.