Find R4x dA\displaystyle \int\int_{{R}}{4}{x}\ {d}{A} over the region R={(x,y)0x4,0y5}\displaystyle {R}={\left\lbrace{\left({x},{y}\right)}{\mid}{0}\le{x}\le{4},{0}\le{y}\le{5}\right\rbrace} by identifying it as the volume of a solid