What is the GCF 16 24 40

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 .

Solve the equation - 2(m -30) = -6m a-15 b-13 c-8 d8

If a || b and b | y, then ? a) x || y. b) x | a. c) a | y. d) a || y.