how to write an equation in the form p=mx+b that relates price to number of phones sold

Question

A wireless franchise determines that if it sells the iphone 5 for the list price of p=$649, it can sell x=100 phones. They find if they lower the price to p=$629, they can sell x=140 phones. Assuming the relationship is linear, write an equation of the form p=mx+b that relates the price to the number of iphones sold

Answer 1

(649,100)

(629,140)

40/-20 = -2

intercept = y - mx = 100 - (-2)(649)

= 100 + 1298

= 1398

y = -2x + 1398