The horizontal blue line marks the level at which the emission C = 200 g/km.
If you go vertically down at the points where that line intersects with your graph, you find the velocity-values v1 and v2 for which C(v1) and C(v2) = 200 g/km.
Here, v1 = 38 km/h and v2 = 112 km/h.
It's easier to calculate this directly, but you need to know how to do this.
The equation to solve is
2
200 = 0.045 v - 6.75 v + 393
This simplifies to
2
0.045 v - 6.75 v + 193 = 0
The solutions to this equation are
6.75 - sqrt( -4*0.045*193 + 6.75^2 )
v1 = ------------------------------------ = 38.44
2*0.045
6.75 + sqrt( -4*0.045*193 + 6.75^2 )
v2 = ------------------------------------ = 112.55
2*0.045
Rounded to km, we find again v1 = 38 km/h and v2 = 112 km/h.