How is the equation for a vertical line expressed?

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 .

What is the equation of a vertical line passing through the point (−5, −1)? y = −5 y = −1 x = −5 x = −1

Given f(x)= 9x+1 and g(x)=x^3, choose the expression (f*g)(x)