Chemical formula of al^3+ and co3^2-

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 

Must be negative.substance increases