Now that you see how it works, let's take another look at the size thing:

B times A = answer ... 3 x 2 times 2 x 2 = 3 x 2 ... the 2's are matched ... the answer matrix will be a 3 x 2

Could we have multiplied  A times B ?

Check the sizes:

A times B ... 2 x 2 times 3 x 2 ... Nope, the 2 and the 3 don't match!

Look what will happen if we try it:

A = [ row 1: 0 , 9  row 2: 8 , -6  row 3: 4 , 7 ] ... B = [ row 1: -1 , 5  row 2: 2 , -3 ] ... there's no partner for the 4!

This just doesn't match up!

Ya can't do it!