F(x)=|x-6| written as piecewise function

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 much dextrose 70% must be mixed with dextrose 5% to prepare 1,000 ml of dextrose 20%?

How much orange juice do you need to make 2 servings of lemony orange slush?