The slope (change in y/change in x) of the line is (1560 - 1200)/(52 - 40), which is equal to 30. We can plug this into the equation y = mx + b using either the point (40, 1200) or (52, 1560). The slope is represented by m and the y - intercept by b. Therefore, the two possible equations you could use are:
1200 = 30(40) + b
or 1560 = 30(52) + b
Using either equation, you'll get that b is equal to 0, which makes sense in the context of the problem, since it's reasonable to expect that the labor expenses would be 0 for 0 hours worked.