If the standard deviation of the numbers 2,3,a and 11 is 3.5, then which of the following is true

standard deviation of number 2,3, a and 11 is 3.5
$\begin{array}{l}(3.5{)}^2=\sum \frac{{\mathrm{x}}_{\mathrm{i}}^2}{4}-{\left(\begin{array}{c}-\\ \mathrm{x}\end{array}\right)}^2\\ (3.5{)}^2=\frac{4+9+{\mathrm{a}}^2+121}{4}-{\left(\frac{2+3+\mathrm{a}+11}{4}\right)}^2\end{array}$
On solving , we get 3a² - 32a + 84 = 0