Find the complex square root of $7+6sqrt2i$

Find the complex square root of

$$7+6\sqrt2i$$
giving your answer in the form $x+iy$ where $x$ and $y$ are real.

The answer I have gotten is $-23+84\sqrt2i$
by squaring $7+6\sqrt2i$,
but the “correct” answer is $\pm(3+i\sqrt2)$.

Can you explain how I got this wrong?