The value of y is directly proportional to the value of x. If y = 55 when x = 120 what is the value of y when x = 60?

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 .

Acredit card advertises an annual interest rate of 23%. what is the equivalent monthly interest rate?

How do i simply this expression (quadratic formula basis) on a ti-84 or normal calculator?