Solved matrix equation- verification

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Solving this matrix equation $\det \begin{vmatrix} x & 3+x & 3+x\\ 2 & x+3 & 6\\ 3 & 4 & x+6 \end{vmatrix}$ I got stuck in this polynomial equation: $x^3+4x^2-16x+15=0$ This seems wrong to me because the value I got for x is kinda wrong and probably I may have use a wrong approach to get here. Pls I need a hint or guide to proceed