The value of $\text{'}\mathrm{x}\text{'}$ for which the angle between $\overline{\mathrm{a}} = 2{\mathrm{x}}^2\overline{\mathrm{i}} + 4\mathrm{x}\overline{\mathrm{j}} + \overline{\mathrm{k}}$ and $\overline{\mathrm{b}} = 7\overline{\mathrm{i}} — 2\overline{\mathrm{j}} +\mathrm{x}\overline{\mathrm{k}}$ is obtuse and the angle between $\overline{\mathrm{b}}$ and Z-axis is acute and less than $\mathrm{\pi }/6$ ,are