Just simplify the given equation
y = 7.2 + 4(x-2)
y = 7.2 + 4x - 8
y = 4x - 0.8
Now they want to know the answer if X = 2
y = 4(2) - 0.8
y = 8 - 0.8
y = 7.2
To determine a percentage difference, simply divide the difference of the two numbers by the original. For instance, 82-65.60=16.4 16.4 is 20% of 82, so the discount is 20% (16.4/82)
For answers, please refer to the picture attached