Friends, couple anyone explain this multiple integral sum?

, double intregration kare ho to ? as there is only dx and dy , and nothing extra function , it will indicate the area enclosed by the surface , here it is a rectangle , but if ...

it's a simple integration of function f(x,y) with x and y... and the given D gives the limiting values of x and y... just simply do the integration of f(x,y), you will automaticall...