Scalar Multiplication of a Matrix

Q. If $A=\left[\begin{array}{cc}0& 2\\ 3& -4\end{array}\right]$ and $kA=\left[\begin{array}{cc}0& 3a\\ 2b& 24\end{array}\right]$, then the values of k, a, b are respectively.

1. -6, -12, -18
2. -6, 4, 9
3. -6, -4, -9
4. -6, 12, 18

Q. Let $S$ be the set which contains all possible values of $l,m,n,p,q,r$ for which
$A=\left[\begin{array}{ccc}{l}^2-3& p& 0\\ 0& m^2-8& q\\ r& 0& n^2-15\end{array}\right]$ be a non singular idempotent matrix. Then the sum of all the elements of the set $S$ is