Now you need to find out: what is the speed v0 that yields the lowest possible emission C? And how much emission would that be?

You can read an approximate solution directly from the graph. If you know how to find minima and maxima, you can compute the answer directly, using the equation.

Enter the speed v0 here (rounded to km/h):

Enter the lowest possible emission C(v0) here (rounded to g/km):