A clothing business finds there is a linear relationship between the number of shirts,n,it can sell and the price,p, it can charge per shirt. In particular, historical data shows that 9000 shirts can be sold at a price of $78, while 10000 shirts can be sold at a price of $70. Give a linear equation in the form p=mn+b that gives the price p they can charge for n shirts.

I cannot seem to solve for an equation as in p=mx+b