PLEASE PLEASE HELP REALLY QUICK❤️❤️

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 .

What adds to be the bottom number but also multiplies to be the top number with 8 on the bottom and 15 on top

What is the scale factor of two similar pyramids with volumes of 64 cubic feet and 8 cubic feet