When it says x and y are directly related it means that x is multiplied by something to get y. To solve you divide y by x which is 840 divided by 6. Which is 140 then you multiply and solve so the y value for 5 is 700. The x value for 420 is 3 and the y value for 4 is 560.
Well, given that a quarter equals 5 nickels, we can reason that, in order to get 12 coins, we would have to get 10 nickels (equals 50 cents) and 2 quarters (the other 50 cents, to make a dollar).
You didn't say whether you needed this expressed in a mathematical formula, but knowing the answer already, it shouldn't be hard to come up with one if necessary.
Hope this helps.
This type of parabola opens either to the left or to the right. The negative makes it open to the left.
