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2a+21=5a-9+3a
We move all terms to the left:
2a+21-(5a-9+3a)=0
We add all the numbers together, and all the variables
2a-(8a-9)+21=0
We get rid of parentheses
2a-8a+9+21=0
We add all the numbers together, and all the variables
-6a+30=0
We move all terms containing a to the left, all other terms to the right
-6a=-30
a=-30/-6
a=+5
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