The basketball gym at Dustin's school was built in the 1940s and needs a lot of repairs. So, the school is asking families for donations to build a new gym. Dustin convinced some local businesses to match a fraction of the families' donations. There is a proportional relationship between the amount of money a family donates (in dollars), x, and the matching contribution (in dollars) from local businesses, y. When a family donates $12, the businesses contribute $9. Write the equation for the relationship between x and y.