passes the vertical line test to make sure it is indeed a function, and
also a horizontal line test to make sure it is it one-to-one.
In other words,
The function will be one-to-one if it passes the vertical line test, and also if the horizontal line only cuts the graph of the function in one place.
The reason is that there must be only one x-value for each y-value.
Given the function
Have a look at the attached graph.
The red portion represents the graph of the function .
The green portion represents the graph of x=2 which is basically a vertical line test. Vertical line indicates that it cuts the cuts the graph of the function in one place. So it is clear that is indeed a function.
The blue line represents the graph of y=9, which is basically a horizontal line test. Horizontal line indicates that it cuts the cuts the graph of the function in one place. So it is clear that is a one-to-one function, as there is only one x-value for each y-value.
Therefore, we conclude that the function is one-to-one.
48 gallons... since the first shower head gives 9 gallons per minute you multiply 9 by 8 then you use the equation and multiply 3 by 8 then you subtract the two shower heads