The points plotted in the coordinate plane represent the possible numbers of adult and child tickets purchased for an event, totaling $54.00 in revenue. The price of each adult ticket is the same and the price of each child ticket is the same. The graph shows the number of adult tickets on the x-axis and the number of child tickets on the y-axis, and the plotted points include $(0,18),(3,12),(6,6),$ and $(9,0)$. What is the price, in dollars, of 1 child ticket?