Question

Subtract $7x$ from $9y$

Answer 1

Hint: Subtraction is the operation of finding out the difference between two numbers. Read the question carefully and observe which number is to be subtracted from which number because sequence of numbers matters in case of subtraction.

Complete step-by-step answer:
The operation of finding out the difference between two numbers is called subtraction. It is also called minus. It is denoted by a negative sign, ( – ).
Subtracting $a$ and $b$ can mean $a-b$ or $b-a$ .
In the same way, the difference between $a$ and $b$ can mean $a-b$ or $b-a$ .
But, subtraction of $a$ from $b$ , specifically means you have to minus $a$ from $b$ i.e. $b-a$
By using above logic, we can say that,
Subtract $7x$ from $9y$ means, we have to do the subtraction of $7x$ from $9y$ .
Mathematically, it can be written as
 $9y-7x$
Expression written above is our answer.
So, the correct answer is “ $9y-7x$ ”.

Additional information:
Subtraction is not commutative. i.e.
 $a-b\ne b-a$
For example:
 $\Rightarrow 1-3=-2\ne 2=3-1$
Subtraction is not associative. i.e.
 $\Rightarrow (a-b)-c\ne a-(b-c)$
For example:
 $\Rightarrow (1-2)-3=-4\ne 2=1-(2-3)$
So, the correct answer is “Option C”.

Note: Since we know that subtraction is not commutative. i.e. $a-b\ne b-a$ . You need to be careful while deciding which term should be subtracted from which term. So read carefully and write the answer only after you understand the sequence in which you have to write the terms. In case of addition we do not worry about the sequence of terms as addition is associative.