Aparabola intersects the x=3 and x=9 what is the x-coordinate of the parabolas vertex?

You can use the two numbers in the ratio to apply onto a problem. Finding the unit rate helps. 
For example, when you have 3 bananas for every tomato, that means that for each tomato you buy, you buy 3 bananas. When you compare the two, bananas to tomatoes, it can help solve problems.