Find the missing number so that the equation has infinitely many solutions._x+12+x=–4x+12

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 .

1mile equals approximately 1.6 kilometers. which best approximates the number of miles in 6 kilometers?

1: 4 if the wew 35 surfboards at the beach how many were short boards?