The reason of A and B in the proof is Associative property of Multiplication.
What is associative property of multiplication.
The associative property of multiplication posits that in a situation whereby multiplication is taking place amongst more than to integers, it doesn't matter which integers or variables are being multiplied first.
In essence, the brackets or parentheses is used in rearrangement of the multiplication problems. i.e. (a × b) × c = a × (b × c).
Here;
= 12 × [j × 2 × (3 × k)]
= 12 × [j × (2 × 3) × k]
This is an associative property of multiplication.
Also;
= 12 × ( 6 × jk)
= (12 × 6) × jk
This is also an associative property of multiplication.