In this case, the question is asking about Km and we know that Km is equal to the half of Vmax. So,

Vmax=2 (because is the higher number)

1/2 Vmax= 1

So the concentration is between 0.2 to 0.5.

So from the definition of Michaelis-Menten Kinetics,
Km = [S] at 1/2 Vmax

For this problem Kmax = 2.0 because that's the value that the velocity maxed out at, even when the substrate [Guanine] was increased 10x.

Back to the definition above, we want to find the amount of substrate, [Guanine] at 1/2 Vmax, when Vmax = 2.0.

Since [Guanine] = 0.5 when velocity = 1.1 and [Guanine] = 0.2 when velocity = 0.6, the [Guanine] when velocity = 1.0 must be between 0.2 and 0.5.