An experiment is performed to investigate the tensile elongation of a structural tendon under load. The axial elongation $x$ of the tendon when a tensioning force $P$ is applied is given by the relation:

$$x=\frac{\gamma P}{E}$$

where $E$ is the Young modulus of the material of the tendon and $\gamma$ is a structural coefficient of the tendon assembly. A graph of axial elongation $x$ (in $mm$) against the applied tensioning force $P$ (in $kN$) yields a straight line through the origin. From the line of best fit, a force of $P=75.0kN$ corresponds to an axial elongation of $x=1.50mm$. Given that the Young modulus of the tendon's material is $E=120GPa$, deduce the order of magnitude of $\gamma$ in $m^{-1}$.