Can someone give me the answer and explain it to me? I honestly don't even get this-

The graph shows the number of gallons of white paint that were mixed with gallons of yellow paint in various different ratios:

Graph titled Mixing Paint, horizontal axis labeled Yellow Paint in gallons, and vertical axis labeled White Paint in gallons is shown. The horizontal scale is 0 to 24 in increments of 3. The vertical scale is 0 to 168 in increments of 21. The plotted points are 3, 21 and 6, 42 and 9, 63.

The number of gallons of white paint mixed with 1 gallon of yellow paint is ______.