Solve for a
a=\frac{S-b}{3}
Solve for S
S=3a+b
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3a+b=S
Swap sides so that all variable terms are on the left hand side.
3a=S-b
Subtract b from both sides.
\frac{3a}{3}=\frac{S-b}{3}
Divide both sides by 3.
a=\frac{S-b}{3}
Dividing by 3 undoes the multiplication by 3.
Examples
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Simultaneous equation
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Limits
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