Find $\displaystyle A^2$

a) A= $\displaystyle \begin{bmatrix} 1 & 3 & 5 \\ 2 & 4 & -3 \end{bmatrix} $

b)A= $\displaystyle \begin{bmatrix} 1 & 2 & 3 \end{bmatrix} $

c) A= $\displaystyle \begin{bmatrix} 1 \\ 2 \\ 3 \end{bmatrix} $

I don't understand how to calculate the power of a matrix.

Can you show me step by step on how to do one of them? Thanks