Solve the following:
Add= $ 7xy + 5yz - 3zx,4yz + 9zx - 4y, - 3xz + 5x - 2y $

Hint: In order to add the three expressions together, first identity all the like terms in the equation and write them together. Now combine all the like terms with their respective operative to simplify the expression.

Complete step-by-step answer:
We are given three mathematical expression $ 7xy + 5yz - 3zx,4yz + 9zx - 4y, - 3xz + 5x - 2y $ containing variable $ x,y,z $
To find the addition of all three expression, lets first them in the equation with including the proper addition $ ' + ' $ symbol between them, we get
Add= $ \left( {7xy + 5yz - 3zx} \right) + \left( {4yz + 9zx - 4y} \right) + \left( { - 3xz + 5x - 2y} \right) $ ---(1)
The expression above contains many like terms. To combine the like terms and to write them together, we should use what are like terms. So,
Like terms are terms which have the same variable.
Let’s write all the like terms in the equation (1) together before combining them
 $
   = \left( {7xy + 5yz - 3zx} \right) + \left( {4yz + 9zx - 4y} \right) + \left( { - 3xz + 5x - 2y} \right) \\
   = 7xy + 5yz + 4yz - 3zx + 9zx - 3zx - 4y - 2y + 5x \;
  $
Now combine all the like term with their respective operations
 $ = 7xy + 9yz + 3zx - 6y + 5x $
ADD $ = 7xy + 9yz + 3zx - 6y + 5x $
Therefore, ADD $ = 7xy + 9yz + 3zx - 6y + 5x $ .
So, the correct answer is “7xy + 9yz + 3zx - 6y + 5x”.

Note: 1.Mathematical equation : A Mathematical equation can be defined as the mathematical statement which contains an equal symbol $ = $ in between two algebraic expressions that share the same value .
A algebraic expression can contain any number of variables generally we take 2-3 variables
Let assume a expression
 $ 5x + 9 = 24 $
It is a mathematical equation having LHS (Left-Hand-Side) equal to RHS (Right-Hand-Side)
Where $ x $ is the variable
5 is the coefficient of variable $ x $
And $ 24,9 $ are the constants
2. $ 2x + 98 + 78y $ is not a mathematical equation because it does not contain equality $ = $ symbol .
It is only a mathematical expression.
3.if the two terms having variable as $ xy $ and $ yx $ respectively , then they are also like terms as $ xy = yx $