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Start with the given equation

Square both sides

Distribute.

Subtract

from both sides.

Combine like terms on the right side.

Divide both sides by

to isolate

.
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Answer:
So the answer is

which in decimal form is

.
Check:

Start with the given equation

Plug in

Add

Take the square root

Multiply

Reduce. Since the equation is true, this verifies our answer.