Determine the numerical value of y=(a+1)(b+1), knowing that ab= -6 and a+b=1

Y=(a+1)(b+1) 
ab=-6
a+b=1
a=1-b

(1-b)b=-6
-b^2+b+6=0
Δ=25, so b=3 or b=-2

Case 1: b=3, a+b=1, a+3=1, a=-2
y=(-2+1)(3+1)=-4
Case 2: b=-2, a+b=1, a-2=1, a=3
y=(3+1)(-2+1)=-4

So the value of y=(a+1)(b+1)=-4

The answer for you question is b