In my point of view, the simple thing to do here is just to exchange the 3+ and the 2- and you will get:
Al2(CO3)3
The logic behind this is that when you have 2 Al^3+, it makes an overall charge of 6+. Additionally, when you have 3 CO^2-, it also makes an overall charge of 6- (because 3 X 2 = 6). Therefore, 6+ and 6- "cancelled" out, and you will have a neutral charge compound.
I hope you understand
