Solve for a
a=\frac{b}{3}
Solve for b
b=3a
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-3a=-b
Subtract b from both sides. Anything subtracted from zero gives its negation.
\frac{-3a}{-3}=-\frac{b}{-3}
Divide both sides by -3.
a=-\frac{b}{-3}
Dividing by -3 undoes the multiplication by -3.
a=\frac{b}{3}
Divide -b by -3.
b=3a
Add 3a to both sides. Anything plus zero gives itself.
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