A Mohr-Coulomb envelop between shear stress, $\tau$ and normal stress, $\sigma_n$ of a sandstone rock is given as

$\tau = 7.5 + 0.84 \sigma_n \quad (\text{unit of stresses is MPa})$

A sandstone sample is tested in triaxial mode with confining pressure of 5.0 MPa. The value of the shear stress, $\tau$ in MPa at the failure, is_____. (round off up to 2 decimals)