Find the value of $m+k$ if subtracting the first polynomial from the second $6a{b}^2-6ab+12a^{2}b \mathrm{and} 19a{b}^2-12a^{2}b$ gives $ma{b}^2-24a^{2}b+kab$

Subtract $\left(x^3+2x^2-x+9\right)$ from$\left(-2x^3-5x^2+7x+2\right)$. If the difference is in the form $-3x^3-k{x}^2+8x-k$, then find the value of $k$.

If the simplified form of$-19a{x}^3+12ax{z}^2-17b{y}^{2}z+8a{x}^3-19b{y}^{2}z+2ax{z}^2$ is $Pa{x}^3+Qax{z}^2+Rb{y}^{2}z$,  then find the value of $P+Q+R$.

Find the value of $k$ if multiplying $21x^{3}y{z}^2 \mathrm{by} 9x$ gives $k{x}^{4}y{z}^2$

Find the value of $k$ if multiplying $\left(\frac{21}{3}x{y}^2-\frac{7}{2}{x}^{2}y+8\right) \mathrm{by} \left(\frac{3}{2}x{y}^2-9\right)$ gives $\frac{k}{2}{x}^2{y}^4-\frac{k}{4}{x}^3{y}^3-51x{y}^2-\frac{63}{2}{x}^{2}y-72$