A function machine is given as:

If the output is 8, calculate the input.
This is done by working out the inverse function.
Working backwards, we must work out what mathematical operation undoes the original function.
The opposite of ‘Subtraction’ is ‘Addition’.
The original function can therefore be reversed as follows:

Therefore inserting 8 as the output gives

Therefore if 8 is the output, 11 was the input.

In Subtract 3 Out