Making 'a' the subject of the equation: a = + or - the square root of (5n+7)/D

Same answer I gave you last time you asked this exact same question

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5n = Da² - 7

Add 7 to both sides

5n + 7 = Da²

Divide both sides by D

(5n + 7) / D = a²

a² = (5n + 7)/ D

Take the square root of both sides

a = ±√ [ (5n + 7) / D ]               D ≠ 0

5n = Da² - 7

5n + 7 = D a²

(5n + 7) / D = a²

± √((5n + 7) / D) = a