$f=\{(-1,0),(2,5),(0,-4)\}\Rightarrow {{D}_{f}}=\{-1,2,0\}$ $g=\{(0,-1),(2,-5),(1,3)\}\Rightarrow {{D}_{g}}=\{0,2,1\}$ ${{D}_{f+g}}={{D}_{f}}\bigcap {{D}_{g}}=\{-1,2,0\}\bigcap \{0,2,1\}=\{0,2\}$ $f+g=\{(0,-4+(-1)),(2,-5+5)\}=\{(0,-5),(2,0)\}$