The first equation is a constant. There is no input necessary, and the output remains 5.
The second equation is also a constant. The output is 4.
In the third equation, y depends on what value of x you use. x is the input and y is the output.
In the fourth equation, x depends on what value of y you use. The way it is written, y is the input and x is the output.
In the last equation, either x or y could be the input. The other variable would change depending on which variable you replace with a number.
