Solve for a
a=-\frac{D}{2}
Solve for D
D=-2a
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D=-2a
Combine -3a and a to get -2a.
-2a=D
Swap sides so that all variable terms are on the left hand side.
\frac{-2a}{-2}=\frac{D}{-2}
Divide both sides by -2.
a=\frac{D}{-2}
Dividing by -2 undoes the multiplication by -2.
a=-\frac{D}{2}
Divide D by -2.
D=-2a
Combine -3a and a to get -2a.
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