Please just help with the too half

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 .

Find the greatest common factor of the followig monomials 21u^5v,3u^4v^6, and 3u^2v^5

Ineed i been stuck on this question since yesterday