The Pool Fun Company did a market survey to decide the pricing of Fun Noodles, a newly developed product. It is estimated that at $3 each, 10,000 Fun Noodles will be sold per day during the summer. Raising the price to $5 will cause the sales to fall to 7000 Noodles per day. Find an equation describing the relationship between x, the price per Fun Noodle and y, the number of Fun Noodles sold per day during summer, assuming that it is linear.