Solve for d
d=-3
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21=8d-18d-9
Use the distributive property to multiply -9 by 2d+1.
21=-10d-9
Combine 8d and -18d to get -10d.
-10d-9=21
Swap sides so that all variable terms are on the left hand side.
-10d=21+9
Add 9 to both sides.
-10d=30
Add 21 and 9 to get 30.
d=\frac{30}{-10}
Divide both sides by -10.
d=-3
Divide 30 by -10 to get -3.
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