{\displaystyle {\begin{aligned}\iint _{D}c\;dA&=\int _{3}^{6}\int _{2}^{4}c\;dxdy\\&=c\int _{3}^{6}\int _{2}^{4}dxdy\\&=c\;A(D)\\&=c\;(3\times 2)=6c\end{aligned}}}