Help me find the value to X

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 height of a square pyramid that has the volume of 32 ft.³ and a base lengthof 4 feet

Which equation expression the relationship between the number of boxes b and the number of cans c and the proportion is 48