$1.)$ Multiply the following matrices $($make sure to state the size of each and if they are compatible$)$:

$\left[\begin{array}{cc}3& 5\\ 1& 9\\ 4& 2\\ 1& 6\end{array}\right]\left[\begin{array}{c}5\\ 3\end{array}\right]=$

$2.)$ Use Gauss Elimination to solve $Ax=b$

$2x_1-3x_2+7x_3=52$

$-10x_1+6x_2+11x_3=-2$

$x_1+4x_2+8x_3=64$

$3.)$ Find determinant $|A|$

$A=\left[\begin{array}{ccc}cos& 0& sin\\ 0& 1& 0\\ -sin& 0& cos\end{array}\right]$