It means there are two chlorine molecules and there is only one magnesium molecule (because no subscript really just means there is an invisible 1 that isn't listed).
As it is a position time graph
So, it's slope represents velocity
During last 15 seconds
it passes through (5,17) (20,2)
Slope = -15/15 = -1
So Velocity= -1 m/s
The final speed is 3 m/s
Explanation:
We can solve this problem by using the law of conservation of momentum. In fact, in absence of external forces, the total momentum of the two spaceships must be conserved. So we can write:

where
is the mass of each spaceship
is the initial velocity of spaceship 1
is the initial velocity of spaceship 2
v is the combined velocity of the two spaceships after the collision
Solving for v, we find

So, their final speed is 3 m/s.
Learn more about conservation of momentum:
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