The function given is $f(x) = -3x + 20$. You are asked to evaluate $f(8)$.

We can substitute $x = 8$ into the function and simplify:

$f(8) = -3(8) + 20 = -24 + 20 = -4$

So, $f(8) = -4$.

Would you like further details or have any other questions? Here are some related questions to explore:

1. What would be $f(-5)$ for the same function?
2. How do you determine if a function like this is linear?
3. What is the slope of the function $f(x) = -3x + 20$?
4. Can you find the x-intercept of this function?
5. What is the y-intercept of the function $f(x)$?

Tip: When evaluating a function, always substitute the input value for $x$ and follow the order of operations (PEMDAS).