In the 2 x 2 case you have four unknowns (a,b,c and d) and six equations. So you can obtain values for the four unknowns. But in the 3 x 3 case you have nine unknowns and only eight equations. So you cannot obtain values for all the unknowns, a,b,c,d,e,f,g,h and i. Similarly I hope you can think about the 9 x 9 case.
Hope this answers all your questions.