Solve for a
a=\frac{1-x}{21}
Solve for x
x=1-21a
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21a=1-x
Swap sides so that all variable terms are on the left hand side.
\frac{21a}{21}=\frac{1-x}{21}
Divide both sides by 21.
a=\frac{1-x}{21}
Dividing by 21 undoes the multiplication by 21.
-x=21a-1
Subtract 1 from both sides.
\frac{-x}{-1}=\frac{21a-1}{-1}
Divide both sides by -1.
x=\frac{21a-1}{-1}
Dividing by -1 undoes the multiplication by -1.
x=1-21a
Divide 21a-1 by -1.
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