19x^2-91y^2=1991

19х:2-91y:2 = 1991

(19x - 91y): 2 = 1991

19х:2 = 1991:(91у:2)

1991 : (-91у) : 2-91 = 1991

91у : 2 =1991-1991+91

91y : 2 = 91

40,5у = 91

у= 91:40,5

у= 2