Help me pls I dint understand

Will give brainliest summations with algebraic expressions image shown

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 .

At the newest animated movie, for every 9 children, there are 4 adults. there are a total of 39 children and adults at the movie.