2
C(v) = 0.045 v  - 6.75 v + 393

This is a formula of the type

          2
y(x) = a x  + b x + c

The question is, what is the emission C in g/km at a speed of v = 50 km/h?

In other words, what is y(50)?

Let's compute:

            2
y(50) = a 50  + b 50 + c = 112.5 - 337.5 + 393 = 168

So the answer is 168 g/km.

Another question