What is the inverse of the function f(x) = 1/4x - 12?

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 .

How do you solve a question like this?

Olga used 100 ounces of flour to make 225 muffins. how many ounces of flour will be used to make 300 muffins?