The sum of p and q is p + q.
The product of p and q is pq.
So, when (p+q) is subtracted from pq,
the final expression will be pq - (p + q).

The terms in an expression are separated by operator + (addition). Therefore, terms of the expression 4pq + p - q - r are 4pq, p, -q, and -r.

Terms containing the same variables raised to the same power are called as like terms.
$\text{Here, 3x, 5x and 9x are like terms.}$
$\text{Similarly, 7y and -4y are like terms.}$

An algebraic expression which consists of only one non-zero term is called a monomial. Examples of monomials are as follows:

a) 2xy is a monomial, but it has 2 variables, 'x' and 'y'
b) 3x + pq -pq is nothing but 3x, which is a monomial, with only one variable, that is 'x'.
c) 5 is a constant
d) 3x +y is a binomial, as it has two terms

So, according to the given question, 3x+ pq - pq = 3x,   is a monomial in one variable.

The pattern in the above case is,
$1=1^2$,  (1st term; n=1)
$4=2^2$,  (2nd term; n=2)
$9=3^2$   (3rd term; n=3)
and so on. 
Similarly, the term at $n^{th}$ position of the pattern will be $n^2$.

Substituting values of a = 3 and b = 2 in the expression:

$\Rightarrow (3-2)(3^2+(3\times 2\left)\right)$

$\Rightarrow (3-2)(9+6)$ 
$\Rightarrow \left(1\right)\left(15\right)=15$

The value of the expression is 15.

Coefficient is defined as a number or symbol multiplied to a variable or an unknown quantity in an algebraic term. 
The coefficient of the term $x^5$ in expression $x^3-x^5$ is -1,
  $-x^5$ = $-1\times \left(x^5\right)$
$\therefore$ The coefficient of $x^5$ is equal to -1.

A monomial added to/subtracted from a monomial is not always a monomial.

For example, 4x - 5y is not a monomial

4x + 5x = 9x is a monomial

A single term or a combination of two or more terms joined by operators such as plus (+) or minus (-) signs forms an algebraic expression. E.g., 58 - y , 6xy + z  etc.
$So,\frac{1}{x}isanalgebraicexpression.$