The best part of this problem is that it has one unknown variable in the form of "x" and a single equation. So getting to the desired solution becomes very easy.
The equation given in the question is
10 - 5x = 20
- 5x = 20 - 10
- 5x = 10
- x = 10/5
- x = 2
Multiplying both sides with -1 we get
x = -2
So the value of x is -2. I hope the procedure is clear enough for you to understand.