<s>[INST] Find a $2 \times 2$ matrix $A$ that satisfies $A^3 = \begin{pmatrix} -1 & -1 \\ 1 & -1 \\ \end{pmatrix}$ 
Find a $2 \times 2$ matrix $A$ such that
  $$A^3 =
    \begin{pmatrix}
    -1 & -1 \\
    1 & -1 \\
    \end{pmatrix}$$

I'm trying to think about this geometrically. Could this have something to do with a rotation dilation?
 [/INST] There are several ways of tackling this problem, a nice one of which you have already thought of.  Since $\ \begin{pmatrix}-1&-1\\1&-1\end{pmatrix}\ $ is indeed a dilated rotation matrix, namely $\ \sqrt{2}\begin{pmatrix}\frac{-1}{\sqrt{2}}&\frac{-1}{\sqrt{2}}\\\frac{1}{\sqrt{2}}&\frac{-1}{\sqrt{2}}\end{pmatrix}\ $, where $\ R=\begin{pmatrix}\frac{-1}{\sqrt{2}}&\frac{-1}{\sqrt{2}}\\\frac{1}{\sqrt{2}}&\frac{-1}{\sqrt{2}}\end{pmatrix}\ $ is a rotation matrix, then the matrix $\ 2^\frac{1}{6} S\ $ will be an answer for your question if $\ S\ $ is a rotation matrix which rotates vectors through one third of the angle that $\ R\ $ does.  The angle through which $\ R\ $ rotates vectors can be found as the angle between the unit vectors $\ \begin{pmatrix}1\\0\end{pmatrix}\ $ and $\ R\begin{pmatrix}1\\0\end{pmatrix}\ $.
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