Answer:
I believe the answer is d = 32
Explanation:
You should isolate the radical, then raise each side of the equation to the power of its index.
9514 1404 393
d = 32
Add 4, then square, and solve the resulting linear equation.
√(7d +1) = 15
7d +1 = 225 . . . . . . square both sides
7d = 224 . . . . . . . . subtract 1
d = 32 . . . . . . . . . . divide by 7
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