I am having a problem understanding how the chain rule was used on to arrive at in the question below (sections highlighted in red)

Question: The edge of an expanding square is changing at the rate of 2 cm/s. Determine the rate of change of its area at the instant when its edge is 6cm long.

Here is the working:

(state the problem mathematically)

(differentiate with respect to time)

(chain rule - I don't understand how this was arrived at )

Use x = 6cm and =2cm/s (and I don't understand why dx/dt =2. In other words, from reading the question above, how would I know that dx/dt=2?)

Answer: The area of the square is changing at the rate of at the instant when its side is 6 cm long