Find the value of the polynomial $x^{2}-7x+12$ at $x=4$.

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: $3x^2 + 5x - 7 + \frac{8}{x}$

Factorise $4 x^{2}+9 y^{2}+49 z^{2}-12 x y+42 y z-28 z x$.

Factorise the following:
$1-64 a^{3}-12 a+48 a^{2}$

Without actually calculating the cubes,find the value of each of the following:
$(21)^{3} + (15)^{3} + (-36)^{3}$