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