What is the vertex of the parabola in the graph?

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 .

Full year - williamshow many square inches are in 60 square feet? 5 square inches72 square inches720 square inche8.640 square inches

How are algebraic expressions evaluated using substitution?