Answer
See below
Work Step by Step
If $A, B,$ and $C$ are $n \times n$ matrices
a) $(A+2B)^2\\=(A+2B)(A+2B)\\=A(A+2B)+2B(A+2B)\\=A^2+2AB+2BA+B^2$
b) $(A+B+C)^2\\=(A+B+C)(A+B+C)\\=A(A+B+C)+B(A+B+C)+C(A+B+C)\\=A^2+AB+AC+BA+B^2+BC+CA++CB+C^2\\=A^2+B^2+C^2+AB+BA+AC+CA+BC+CB$
c) $(A-B)^3\\=(A+(-B))^3\\=(A+(-B))(A+(-B))(A+(-B))\\=(A^2-AB-BA+B^2)(A+(-B))\\=A^3-A^2B-ABA+ABB-BA^2+BAB+B^2A-B^3\\=A^3-ABA-BA^2+B^2A-A^2B+AB^2+BAB-B^3$
