There might be something missing in the given equation. Since C is a function of x, the equation must contain x. For sample purposes, let's just assume the equations are:
C(x) = 0.35x for x≤8000
C(x) = 0.75x for x≥8000
The domain is from 0 to 42000. This is the x-axis as shown in the picture. Sample calculations are shown on the left of the graph. The blue graph is for the function C(x) = 0.35x for x≤8000, while the orange graph is for the functionC(x) = 0.75x for x≥8000.The graph break is located when x = 8000.