The function you provided is:

$f(-3) = 6 - 7x$

This means that when the input is $x = -3$, the output of the function is:

$f(-3) = 6 - 7(-3)$

Now, I'll calculate this expression.

$f(-3) = 6 + 21 = 27$

Thus, the output is $f(-3) = 27$.

Would you like more details or have any questions?

Here are five related questions to expand your understanding:

1. What is $f(x)$ if $f(x) = 6 - 7x$?
2. How do you find the value of a function for a given input?
3. Can you graph the function $f(x) = 6 - 7x$?
4. How do you solve $f(x) = 0$ for $f(x) = 6 - 7x$?
5. What is the slope of the line represented by $f(x) = 6 - 7x$?

Tip: When plugging negative numbers into functions, remember to pay attention to the signs when multiplying!