Perimeter of square = 40 units
                    $4\times$ side = 40
                      $4\times 2x$ = 40
                             $8x$ = 40

So, $x$ = 5 units.

So, the length of each side is $5\times 2$ =  10 units.

5ab =5×3×2=30

The 'term' of an algebraic expression is either a single number or variable, or number and variables multiplied together.
When such terms are added, it forms an algebraic expression.
So, $2x$ and $\left(\frac{-1}{2}\right)$ are $terms$ of the algebraic expression $(2x-\frac{1}{2})$.
Here, we should notice that ‘minus’ is associated with the terms.

Solve groups:
= x²(2+7)+y(3+6)+5z²+10p
= 9x²+9y+5z²+10p

A constant is a number on its own which has a fixed value whose value does not change.
In the given options the values  of  'xy', '4x+6y' and 'pxy' are not fixed and can change depending on which value 'x' and 'y' takes, but '7' is a fixed value.
So '7' is a constant and others are variables.