Solve each system of equation by substitution

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