1) y^2 -7y-18=0
y= -2 y = 9
значит x^2 = -2 корней нет
x^2 = 9 x = =- 3
2) y^2 -13y+36=0
y = 4 y = 9
x = +-2 x =+-3
3) (a-5) ^2 -2(a-3) -4 =0
a^2 -10a + 25 -2a +6 -4 = 0
a^2 -12a + 27 =0
a = 3 a= 9
x^2 -2x = 3
x^2 -2x -3=0
x= -1 x = 3
x^2 -2x =9
x^2 -2x -9 =0