x+y=11
xy=128
x=11-y, so (11-y)y=128
11y-y²=128
y²-11y+128=0
use the quadratic formula to find y: y=(11+√391i)/2 or y= (11-√391i)/2
x=(11-√391i)/2, or x=(11+√391i)/2
so either there is an error in your question, or you have two unreal numbers:
(11-√391i)/2 and (11+√391i)/2
