Solve for a
a=\frac{7-b}{2}
Solve for b
b=7-2a
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2a=7-b
Subtract b from both sides.
\frac{2a}{2}=\frac{7-b}{2}
Divide both sides by 2.
a=\frac{7-b}{2}
Dividing by 2 undoes the multiplication by 2.
b=7-2a
Subtract 2a from both sides.
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