Factorise: $49x^2 - 121$

Which of the following expressions is a polynomial? State the reason. If an expression is a polynomial,state whether it is a polynomial in one variable or not.
$5x^2 + 11x - 2\sqrt{x}$

Factorise the following:
$4 x^{2}+20 x+25$

With the help of the remainder theorem,examine whether $x+2$ is a factor of the polynomial $x^{3}+9x^{2}+26x+24$ or not.

Is the following expression a polynomial? Justify your answer:
$\frac{1}{2x}$

Expand the following:
$(-x+2y-3z)^{2}$