How to calculate $\iint_D y\, dx\, dy$, $(D = \{(x, y) : x \le y \le 2x, 0 \le x \le 3\} )$? How to calculate $$\iint_D y\, dx\, dy,$$ where $$D = \{(x, y) \mid x \le y \le 2x, 0 \le x \le 3\} )?$$

This is the first time I have to solve such question, so I have definitely no idea how to do it.

If you can help me HOW to solve it, and not SOLVE IT for me,

I'll be very thankful.

What is the $D$ below the inner integral?

Thanks in advance!
Check that you can apply Fubini's theorem, and integrate first with respect to $y$.
$$\int \int_D ydxdy $$
= $$\int_0^3 \int_x^2xydxdy$$

= $$\int_0^3(x\int_x^2ydy)dx$$

= $$\int_0^3\frac{4x-x^3}{2}dx$$

Now you can do it