Ta có:
M=(2x-y-z)-(x+y+z)+2(x-y-z)-(x-y-z)
=\([2x-\left(y+z\right)]-\left(x+y+z\right)+2[x-\left(y+z\right)]-[x-\left(y+z\right)]\)
=\(\left[2.\left(-2\right)-1\right]-\left(-2+1\right)+2.\left(-2-1\right)-\left(-2-1\right)\)
=-5-(-1)+2.(-3)-(-3)
=-5+1-6+3
=-7