A19 = - 58, a21 = - 164

I took a19=-58 divide it by 19 both side i get a = - 58:19 (note; you can never have a neg. # on the top bring the neg. down to the bottom and divide) now its 58:-19 = - 3.05 or - 3 (1:19) so a = - 3 (1/19) do the same to the other now. answer is a=-7 (17/21)