Solve for a
a=\frac{A-7}{21}
Solve for A
A=21a+7
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7+21a=A
Swap sides so that all variable terms are on the left hand side.
21a=A-7
Subtract 7 from both sides.
\frac{21a}{21}=\frac{A-7}{21}
Divide both sides by 21.
a=\frac{A-7}{21}
Dividing by 21 undoes the multiplication by 21.
a=\frac{A}{21}-\frac{1}{3}
Divide A-7 by 21.
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