Hi there!
The answer is D.
We need to isolate each square root. We can simply do this by adding [tex] \sqrt{4x + 8} [/tex] to both sides of the equation.
When we isolated the square roots, our equation looks like this:
[tex] \sqrt{7x - 7} = \sqrt{4x + 8} [/tex]
Now we can easily square both sides of the equation, which would have been much harder earlier.
~ Hope this helps you!