I’m kinda stupid this might be an easy question but i wasn’t at school sooo

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 .

Complete the solution of the equation. find the value of y when x equals to 6 4x+y=20

Complete the equation of the live through (-1,6) (7,-2)