Ineed so this is so hard and this is not me

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 .

If f(x) = x+8 and g(x) = 4x- 3, find (f+g)(x)

Adam used 1072.4 gallons of water over the course of 7 days. how much water did adam use, on average, each day?