What is the value of k? 5k-3

The two-way frequency table below shows data on playing a sport and playing a musical instrument for students in a class.

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)=2x-6 and g(x)=3x+9, find (f-g)(x)