The graph consists of a straight line
y = ax + bin a coordinate system where the x-axis corresponds to power consumption, and the y-axis corresponds to money to pay.
If the line is drawn correctly, one can read the price of electricity (y-value) for every possible amount of power consumption (x-value).
One point of the line is known already: it corresponds to this month's bill. It is drawn as a green dot.
Your job is to help by adjusting the sliders such that the line is drawn correctly. (You need to know the equation of that line...)