Cummuative law states that a x b = ab and b x a =ab ... this doesnt apply for matrixes...

because its row by column system... if i was to prove this.. i would simply do a [[a,b][c,d]]x[[e,f][g,h]] then do [[e,f][g,h]]x [[a,b][c,d]] show they are not the same

Premultiplication is when you try and get rid of the matrix on one side you times it by the inverse yes... so then you times the other side by the inverse in the same fashion e.g

A x B = X

A^-1xA x B = A^(-1)X

B = A^(-1)X