In the X$-$ray diffractometer, it is observed in the spectrum of a specific sample element that the diffraction angles are $5\mathrm{.}4$ from $(100)$ planes, $7\mathrm{.}6$ from $(110)$ planes, and $9\mathrm{.}4$ from $(111)$ planes, respectively. By giving all the relevant steps and reasons clearly, find out and specify the unit cell crystal structure of the sample element $($that is$,$ determine whether it is a Simple Cubic $($SC$)$ unit cell, Face$-$Centered Cubic $($FCC$)$ unit cell, Body$-$Centered Cubic $($BCC$)$ unit cell or Hexagonal Close$-$Packed $($HCP$)$ unit cell$).($Note: Take the $1^{st}-$order reflection $n=1$ in Bragg's law!$)$