(6 1/2+2 2/5 )+ 2 1/2 using properties of addition

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 .

The parabola y = x² - 4 opens: a.) up b.) down c.) right d.) left

Add the fractions, and simply the result to lowest terms. 2/3 + 2/7= ?