You can put this solution on YOUR website! The idea is to isolate 'a' to one side of the equation -- doesn't matter which side.
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L=2d+pi(a+r)
Start by subtracting 2d from both sides to get:
L-2d = pi(a+r)
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Now, divide both sides by pi:
(L-2d)/pi = a+r
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Finally subtract r from both sides:
((L-2d)/pi) - r = a
