The example diagram shows '9' as an input connected to a diamond containing '3', which then leads to an output of '5' in a circle.

The example diagram shows '9' as an input connected to a diamond containing '3', which then leads to an output of '5' in a circle.
Let $I$ be the input number(s) connected to the diamond, $D$ be the number inside the diamond, and $O$ be the output number in the circle.
For the example: $I = 9$, $D = 3$, $O = 5$.
We need to find a mathematical relationship between $I$, $D$, and $O$.
Let's test the operation $I - D - 1$:

This relationship holds true for the example. So, the pattern is $O = I - D - 1$.

For , the inputs connected to the diamond are '6' and '7'. The number inside the diamond is '5'. The output is '?'.
When multiple numbers are connected as inputs to a single operation in such puzzles, they are typically combined by addition.
So, the combined input $I$ will be the sum of '6' and '7'.
The number inside the diamond $D = 5$.
The output $O = ?$.

Substitute the values of $I$ and $D$ into the pattern $O = I - D - 1$:

The missing number is 7.

The final answer is 7.