Most of the time, the independent variable is located in the horizontal axis (x) and the dependent variable is located in the vertical axis (y), a change in the independent variable causes an influence on the value of the dependent variable, in this case, the dependent variable would be the profit in $ (thousands) and the independent variable is the # of tickets sold (thousands).
The slope of a line (m) is calculated by means of the formula:
Where (x1,y1) and (x2,y2) are points of the line, i this case, the x coordinate of the line represents # of tickets sold and the y coordinate the profit in $.
We can take whatever point from the line, I'll take the points (0,0) and (40,60), then the slope of the line is:
Then, the slope of the line equals 3/2
The slope of this line represents the earnings per ticket