mean (x̄) = \(\frac{1 + x + 1 + 2x + 1}{3}\) = \(\frac{3x + 3}{3}\) = 1 + x \(\begin{array}{c|c} X & (X -X) & (X -X)^2\\ \hline 1 & -x & x^2 \\ x + 1 & 0 & 0\\2x + 1 & x & x^2\\ \hline & & 2x^2\end{array}\) S.D = \(\sqrt{\frac{\sum(x - x)^2}{n}}\) = \(\sqrt{(6)}\) ( given ) \(\sqrt{(6)}\) = \(\sqrt{\frac{2x^2}{3}}\) cancel square roots and cross multiply \(2x^2\) = 18 divide both sides by 2 \(x^2\) = 9 ∴ x = \(\pm\) \(\sqrt{9}\) = \(\pm\)3