How do you turn a ratio into a percentage number

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 .

Solve for x -2.3(r - 1.2 ) = -9.66 enter you answer as decimal.

Which simplifys to a+ b? a-(1-b)+1 -(1-a)-b+1 (a--1) -1)+1