The rate of change is always the same for a straight line no matter the points used.
The equation of a straight line is given by:
y = mx + b;
where y, x are variables, m is the rate of change and b is the y intercept.
In triangle BAC:
Rate of change = change in y(cost) / change in x(pens) = (6 - 0) / (3 - 0) = 2
In triangle BDE:
Rate of change = change in y(cost) / change in x(pens) = (8 - 0) / (4 - 0) = 2
The rate of change is always the same for a straight line no matter the points used.