Please help thank you so much

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 .

If i purchase a product for $79.99 and two accessories for 9.99 and 7.00 how much will i owe after taxes applies 8.75%

How many zeros does this polynomial function, y=(x-8)(x+3)^2