An experiment is performed to investigate the deflection of a horizontal beam supported at both ends. The vertical deflection $s$ at the midpoint of the beam when a mass $m$ is suspended is given by the relation:

$$s=\frac{\eta m}{E}$$

where $E$ is the Young modulus of the material of the beam and $\eta$ is a constant. A graph of vertical deflection $s$ (in $mm$) against the suspended mass $m$ (in $kg$) yields a straight line through the origin. From the line of best fit, a mass of $m=0.40kg$ corresponds to a vertical deflection of $s=32.0mm$. Given that the Young modulus of the beam's material is $E=2.25GPa$, deduce the order of magnitude of $\eta$ in $s^{-2}$.