OCR MEI Core 1 8.04 Binomial Expansion: Fully Expand (2 - 3x)^4

[tallulaaustin]

I'm having confusion. I thought 'a' represented the coefficient inside the bracket. So i thought 'a' must always start at its highest power and finish at its lowest. I have done lots of practise questions on (x+2) ^3 styled ones and answered correctly. But as this video contradicts my rule ( 2 is a constant but is placed from highest power to lowest) i am wondering is it the order of which the subjects are in the bracket? Meaning whatever is first in the bracket is a - therefore should go from high to low. and whatever is second is b - and should go from low to high? It would be most helpful if someone could enlighten me, thank you!