in the equation y = 5x - 25, x is the input and y is the output. You know this because you start with some x value, apply some operations, and you get the answer y. So if -25 is the output, then y=-25. To find the input, just solve for x:
y=5x-25
-25=5x-25
0=5x
x=0
So the input is 0.
