What is the average speed if time = 3 distance = 90

Iam trying to solve a linear equation for the shoe size of someone 84" tall taking into consideration example 1 is 69" tall with a shoe size of 9 and example 2 is 72 inches tall with a shoe size of 11. i understand that the equation should be y=mx+b and that on the graph the answer is (3,2) but i just don't understand how to create the equation to solve for 84-inches and 60-inches tall questions. after two days, i need .

Jacks bank statement shows an account balance of -$25.50. which describes the magnitude, of size, of the debt in dollars

Can you me find the missing length. i attached an image.