I really need help on an equation and i can't figure it out. Found no example like this in my book, nothing.

When Mary sells pumpkins at 4 dollars she sells 500 when she decreases her price to $3 she sells 900 pumpkins. Her cost are $900 fixed and $2 per techinque.

Find the Price Equation, Revenue Equation And Marginal Cost.

I Have no idea where even to start as terrible as that sounds!!!!!(Crying)