Math geometry

Please help

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 f(x)=x^3 and g(x)=2x+7 when g(x)=-5

The holiday party will cost $160 if 8 people attend.if there are 40 people who attend the party, how much will the holiday party cost