2.1
The Unknown Number
An equation is an equality between two sides. Those two sides are divided by an equal sign (=).
The numbers on those sides can be written out if you know what they are.
Here is an example:
Left Hand Side (LHS) 
Right Hand Side (RHS) 
4 + 2 
= 
7  1 
Four plus two is sex 
= 
Seven minus one is also six 
6 
= 
6 
The two sides are equal.
If we leave out a number on either side, we can figure out what that number should be.
Example:
A) 7 + __ = 10
Here you can probably see that it should be 3 which has been left out because
7 + 3 = 10
B) 8  __ = 3
Here you can see that it should be 5 which has been left out because
8 – 5 = 3

In equations instead of using spaces or question marks, you usually use other signs, normally letters for the unknown numbers. Often x, y, or z is used because there are letters not often seen in writings.
3 + x = 9
3 + 6 = 9
We replace the letter x with a sex so that the equality is correct. We can therefore say that in this case x = 6. x is even called a root of the equation. 