Expand: $(3x + 7y)(3x - 7y)$

Which of the following expressions are polynomials in one variable and which are not? State the reason for your answer. If the given expression is a polynomial,state whether it is a polynomial in one variable or not.
$x^{2} + x - 3 + \frac{4}{x}$

Factorise the following:
$8 p^{3}+\frac{12}{5} p^{2}+\frac{6}{25} p+\frac{1}{125}$

What are the possible expressions for the dimensions of a cuboid whose volume is given as $(2x^3 + 15x^2 + 33x + 20)$ cubic units? $(x > 0)$

Classify the following as linear,quadratic,or cubic polynomial:
$5t + 3$

Without actually calculating the cubes,find the value of:
$\left(\frac{1}{2}\right)^{3}+\left(\frac{1}{3}\right)^{3}-\left(\frac{5}{6}\right)^{3}$