y³-3y²+9y+3y²-9y+27-y³+6y = 0
6y+27 = 0
6y = -27
y = -27/6 = -9/2
y = -4.5
a+b=-15+8= -7
a-b=-15-8= -23
-a+b= 15+8= 23
-a-b= 15-8= 7