#### Answer

$-3^{\circ}$

#### Work Step by Step

Here, $T-T_s=(T_0-T_s)e^{-kt}$
This implies that
$39^{\circ}-T_s=(46^{\circ}-T_s)e^{-10k}$ and
$33^{\circ}-T_s=(46^{\circ}-T_s)e^{-20k}$
This implies that
$\dfrac{39^{\circ}-T_s}{46^{\circ}-T_s}=(\dfrac{39^{\circ}-T_s}{46^{\circ}-T_s})^2$
Thus, $T_s=-3^{\circ}$