Simplify equation (2 - a/b) / ( -4 + a x a / b x b )

Seth practice piano for 1 1/3 hours on saturday and 5/6 hour on sunday for much longer did he practice on saturday than on sunday?

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 .

Ineed the answers for this one as well!