[tex]\displaystyle a_1=2;~a_4=-1~~~~~~~~~~~~~~~~~~~~~~a_{2019}=?\\ \\ a_4=-1 \Rightarrow 2+(4-1)r=-1 \Rightarrow 2+3r=-1 \Rightarrow 3r=-1-2 \Rightarrow \\ \\ \Rightarrow 3r=-3 \Rightarrow r=-\frac{3}{3} \Rightarrow r=-1\\ \\ a_{2019}=2+(2019-1)r=2+2018 \cdot (-1)=2-2018=-2016\\ \\ \mathbf{a_{2019}=-2016~(B)}[/tex]
