This equation is true because of the commutative property of addition. In other words, when you add or subtract two numbers, it doesn't matter in which order you do it. So, in this case, it doesn't matter if we first do (p-q) + q - p or (q-p) + a(p-q), the result will be the same.
This equation is true because of the commutative property of addition. In other words, when you add or subtract two numbers, it doesn't matter in which order you do it. So, in this case, it doesn't matter if we first do (p-q) + q - p or (q-p) + a(p-q), the result will be the same.