[tex]a_{n}=a_{1} + (n - 1)\cdot{r}\\\\a_{19}=a_{1}+(19-1)\cdot{r}\\\\-58=a_1+18r\\\\a_1=-18r-58\\\\\\a_{21}=a_1+(21-1)\cdot{r}\\\\-164=a_1+20r\\\\a_1=-20r-164\\\\\\-18r - 58 = - 20r-164\\-18r+20r=-164+58\\2r=-106\\r=-53\\\\a_1=-20r-164\\a_1=-20\cdot{(-53)}-164\\a_1=1060-164\\a_1=896\\\\\boxed{a_n=896+(n-1)\cdot{(-53)}}[/tex]
B
