This is a STILL image.

It is NOT a movie.

Point A iii

Here we can determine the length of BC fairly easily, using Pythagoras' rule:
√(1^2 + 3^2) = √10.
Finding the length of BA1 is less straightforward, but can be done by, for example, using the similar triangles BFE and BA1G (above).
As can be seen, BA1 = 12÷√10 (or 1.2x√10).

In the movie on the next page we move point B only, in much the same way as we had earlier moved point C only or point A only, but this time along a line parallel to AC.