$\displaystyle {18\cdot \left( {17 \over 36} - {5 \over 18} - {7 \over 12} \right)= 18 \cdot \left( {17 \over 36} - {10\over 36} - {21 \over 36} \right) = }$

 $\displaystyle { =18\cdot {-14 \over 36}={-14 \over 2}=-7}$