Answer:
From the graph find the maximum velocity and half it i.e. Vmax/2. Draw a horizontal line from this point till you find the point on the graph that corresponds to it and read off the substrate concentration at that point. This will give the value of Km.
Explanation:
1) Amphoteric substances can react both like an acid or a base depending if they are in presence of a base or an acid.
Zinc hydroxide (Zn(OH)2):
In acid: 
In base: ![Zn(OH)_2 + 2 NaOH \longrightarrow Na_2[Zn(OH)_4]](https://tex.z-dn.net/?f=Zn%28OH%29_2%20%2B%202%20NaOH%20%5Clongrightarrow%20Na_2%5BZn%28OH%29_4%5D)
Aluminium hydroxide (Al(OH)3)
In acid: 
In base: ![Al(OH)_3 + NaOH \longrightarrow Na[Al(OH)_4]](https://tex.z-dn.net/?f=Al%28OH%29_3%20%2B%20NaOH%20%5Clongrightarrow%20Na%5BAl%28OH%29_4%5D)
2) Reactions
a) 

b) 

To do this, just take the given miles and multiply it with the ratio of molecules in the chemical equation to get your answer.