Anytime you forget exponent rules, try writing the whole thing out like this:
= (2a3)4
= (2a3)(2a3)(2a3)(2a3)
= (2aaa)(2aaa)(2aaa)(2aaa)
= (2 * 2 * 2 * 2) * (aaa * aaa * aaa * aaa)
= 24 * a12
= 16a12
And here's the shortcut method:
= (2a3)4
= 24 a(3 * 4)
= 24 a12
= 16a12