Given temperature at 12 noon is 10°C 
The temperature decreases at the rate of  2°C per hour until midnight
From 12 noon to 9 p.m. it is 9 hours
Temperature decrease  $=9\times \left(-2^{\circ }\mathrm{C}\right)=-{18}^{\circ }\mathrm{C}$
$\therefore$ Temperature at 9 p.m. $={10}^{\circ }\mathrm{C}+\left(-{18}^{\circ }\mathrm{C}\right)=-8^{\circ }\mathrm{C}$  .