You substitute in an expression by filling in the desired and accustomed unknown value to the value you're trying to find. An example below can better elaborate what I want to say above. In the equation below, 7 - 2n = 5 the unknown is represented by n moreover, the equation suggest to find the missing to make it equilateral on both sides.
In mathematical expression, the equation then becomes 7 - 2n = 5
The next procedure is to find the unknown.
1. 7 - 2n = 5
2. -2n = 5 - 7
3. -2n = -2
4. n = -2/-2
5. n = 1
Check:
Substitute n = 1
7 - 2 (1) = 5
7-2 = 5
5=5
