Hey there!
In order to find your given output value, all you need to do is plug it in for f(x) and solve for x in the equation. Whatever you get for x will be the input value that corresponds with that output value. After that, you can plug in that input value for x and solve for f(x) and, if it's the same, then the input value is correct.
For example (with an output value of 10):
f(x) = x + 7 (plug in output)
10 = x + 7 (subtract 7 from both sides)
3 = x (input value)
f(x) = x + 7 (plug in input)
f(x) = 3 + 7 (simplify)
f(x) = 10 (output value)
Just plug in whatever output you were given where I put 10 and solve.
Hope this helped you out! :-)