The rule for the input and output values that are represented in the flow diagram below. B) output = u + 6

Input values: $\{-9, -7, -5, -3, -1, 1, 3, 5\}$
Output values: $\{-3, -1, 1, 3, 5, 7, 9, 11\}$

Let's test the difference between output and input for a few pairs:
For input $-9$, output is $-3$: $-3 - (-9) = -3 + 9 = 6$
For input $-7$, output is $-1$: $-1 - (-7) = -1 + 7 = 6$
For input 1, output is 7: $7 - 1 = 6$
For input 5, output is 11: $11 - 5 = 6$

Since the difference is consistently 6, the rule is to add 6 to the input to get the output.
If the input is represented by $x$ and the output by $y$, the rule is $y = x + 6$.

The rule is $\boxed{\text{output = input + 6}}$.

Let the unknown number be represented by a variable, for example, $x$.

The product of 3 and $x$ is $3 \times x$, which can be written as $3x$.

This means we subtract the number ($x$) from the product ($3x$).
So, the expression is $3x - x$.

The correct algebraic expression is $\boxed{3x - x}$.