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