Step-by-step explanation: For a line to be parallel to another line there must be two conditions true.
Condition 1: Both lines must have the same slope (i.e the same x in the form y = mx + b)
Condition 2: Both lines must have different y-intercepts (i.e different b in the form y = mx + b)
Thus, if you are looking for the slope of the line parallel to y = 4/5x - 3, we can look at condition 1, and see that the slope should also be 4/5 for the other line.
cavalieri's principle makes that statement about the volumes, not about the areas.