2. Let $A=\{a \mid-1 \leqslant a \leqslant 2\}$, then the area of the plane point set $B=\{(x, y) \mid x, y \in A, x+y \geqslant 0\}$ is $\qquad$ .
Answer
As shown in the figure, draw the plane point set corresponding to the shape $M B C D N$. Then the area of the shaded part $S=3 \times 3-\frac{1}{2} \times 2 \times 2=7$.