A school typically sells 500 yearbooks each year for $50 each. The economics class does a project and discovers that they can sell 100 more yearbooks for every $5 decrease in price.The revenue for yearbook sales is equal to the number of yearbooks sold times the price of the yearbook.
 
Let X represent the number of $5 decreases in price. If the expression that represents the revenue is written in the form R(X)=(500+ax)(50-bx). Find the values of a and b.