Do you mean square root of(11-x) ?
If so, [square root(11-x)]^2=(x+1)^2
check: SQRT(11-(-5))=SQRT(16)=4 and x+1=-5+1=-4 but SQRT symbol means principal root
(the nonnegative root)
we don't use -5 as a solution
SQRT(11-2)=SQRT(9)=3 and x+1=2+1=3
we use x=2 as a solution