Answer:
Reason:
A square root represents an exponent of 1/2
So
Raising that to the fourth power gets us this,
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Edit: Here's an alternative route
The first line is possible because raising anything to the 4th power is the same as having 4 copies of it multiplied out (eg: 3^4 = 3*3*3*3 = 81).
Then in the second line, I collected the terms into pairs
By line 3, the square root and squaring exponent cancel each other out. This only works if y is not negative.