Here's how to find out:
Take the equation: d/0.2 = 3d + 2.1
Multiply each side by 0.2 : d = 0.6d + 0.42
Subtract 0.6d from each side: 0.4d = 0.42
Divide each side by 0.4 : d = 1.05
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This answer has been changed.
I got d = -0.75 the first time.
Then I came back and changed it.
It's correct now. d = 1.05