What's the y-intercept? y= 2x^2+6x+7

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 .

Tom had a total of $220 and he spent $35 on a basketball ticket . what percent of his money did he have left?

Find the area of an equilateral triangle with a side of 6 inches