Question

What is the Oxidation number of the underlined element in Al2(CO3)3

Answer 1

Jeff, here is a web site by Chemteam that discusses almost any chem subject you wish to discuss. They are excellent and I urge you to copy this link. I can't post the entire link so you will need to add the h t t p s://www. without the spaces. chemteam.info/Redox/Redox.html
That site gives explicit instructions on redox question including how to determine the oxidation state of any material. I don't see anything underlines. For example, however, let's do Al. All compounds are zero total so we know Al is in group 3 so we know the carbonate ion is 2- so we know Al must be 3+ for each Al. Etc. Post your work if you get stuck.

Answer 2

Al2 = 2x, o.n of carbon=+4, o.n of oxygen= -2
2x+3(4)+(-2)3x3=0
2x+12-18=0
2x-6=0
2x=6
x=3

Answer 3

Oxidation number of Al2(co)3