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15a+-15a+20+7a-8a=-19
We move all terms to the left:
15a+-15a+20+7a-8a-(-19)=0
We add all the numbers together, and all the variables
-1a+39=0
We move all terms containing a to the left, all other terms to the right
-a=-39
a=-39/-1
a=+39
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