Answer
$$0$$
Work Step by Step
Our aim is to integrate the triple integral as follows:
$$ Average=\dfrac{1}{2} \times \int^1_0 \int^1_0 \int^2_0 (x+y-z) \space dz \space dy \space dx \\=\dfrac{1}{2} \times \int^1_0 \int^1_0 (2x+2y-2)\space dy \space dx \\=\dfrac{1}{2} \times \int^1_0(2x-1)dx \\=0$$
