Which of the following expressions is a polynomial, state the reason ? If an expression is polynomial, state whether it is a polynomial in one variable or not

$x^{2}+y^{2}+z^{2}+2 x y+2 y z+2 z x$

Write the coefficient of $x^{2}$ in the following polynomials

$\sqrt{3} x^{2}+11$

Verify whether the following are True or False:

$-3$ is a zero of $x-3$

Find the following product :

$(2 x-y+3 z)\left(4 x^{2}+y^{2}+9 z^{2}+2 x y+3 y z-6 x z\right)$

Factorise

$x^{3}+12 x^{2}+39 x+28$

Expand

$(x+3)(x+8)$