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It is about solving simple linear equation in one unknown --- one of simplest problems in Algebra.
Step 1. Subtract 2x from both sides. You will get then
2x + 8 = -12.
Step 2. Subtract 8 from both sides. You will get
2x = -12 - 8, or
2x = - 20.
Step 3. In the last equation, divide both sides by 2. You will get
x = -20/2 = -10.
It is what the problem asks to prove.
The solution / (the proof) is completed.
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To learn the subject (on solving simple linear equations), see the lesson
- HOW TO solve a linear equation
in this site.
This subject is one of the basic Algebra skills.
