C(x) = -5x^2+x
Find c(4)

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 .

Someone answer this check out the image!

Table which ratio is equivalent to 3: 7? 6: 7 3: 21 24: 56 15: 30