[tex]\it \begin{cases}\it 4a+2b=-18|_{:2} \Rightarrow 2a+b=-9 \Rightarrow b = -9-2a\ \ \ \ (1)\\ \\ \it5+a+b=0 \Rightarrow a+b=-5 \Rightarrow b = -5-a\ \ \ \ \ (2)\end{cases}\\ \\ \\ (1),\ (2) \Rightarrow -5-a=-9-2a \Rightarrow -a+2a=-9+5 \Rightarrow a=-4\ \ \ \ (3)\\ \\ \\ (2),\ (3) \Rightarrow b=-5-(-4) \Rightarrow b=-5+4 \Rightarrow b=-1[/tex]
