If the remainder on division $x^3+2x^2+kx+3$ by $x-3$ is $21$, find the value of $k$ and hence find the zeroes of the cubic polynomial $x^3+2x^2+kx-18$.

Solve the equation $x^2-\left(\sqrt{3}+1\right)x+\sqrt{3}=0$ by completing the square method

Solve $3\left(2x+y\right)=7xy; 3\left(x+3y\right)=11xy$ using elimination method.

Find the square root of the polynomial $4+25x^2-12x-24x^3+16x^4$ by division method.