D is the triangle with corners in (0,0), (1,-1) and (3,0).

If we make the substitution: $\displaystyle u=x+y$ and $\displaystyle v=x-2y$ we end up with a triangle E with corners in (0,0), (0,3) and (3,3).

The triangle D is formed by the three lines $\displaystyle x+y=0$, $\displaystyle x-2y=3$ and $\displaystyle y=0$, but I'm not quite sure how you're supposed to think when you transform this to uv coordinates (the E triangle).