Find the rotation matrix that C that transforms the matrix
|0| into |3|
|5| |4|
(the first matrix is 0 5 and the second has 3 4, each vertical)
Another way to do this: Any "rotation" matrix in the plane is of the form
$\displaystyle \begin{bmatrix}a & -b \\ b & a\end{bmatrix}$
with $\displaystyle a^2+ b^2= 1$.
$\displaystyle \begin{bmatrix}a & -b \\ b & a\end{bmatrix}\begin{bmatrix}0 \\ 5\end{bmatrix}=\begin{bmatrix}-5b \\ 5a\end{bmatrix}= \begin{bmatrix}3 \\ 4\end{bmatrix}$
gives you two equations to solve for a and b. Since the lengths of the two vectors, <0, 5> and <3, 5> are the same, $\displaystyle a^2+ b^2= 1$ will automatically be satisfied.