10x - 21 = 7x
add 21 to both sides. You will get
10x = 7x + 21
subtract 7x from both sides. You will get
10x - 7x = 21
3x = 21
divide both sides by 3. You will get
x = 21/3 = 7. ANSWER
Solved.
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- HOW TO solve a linear equation
in this site.