Question 1055331
6===-b==-4==  6==-b
b====0==1===b===0
1===-2===1====1===-2
Need the determinant to be 0.  I added the first and second columns to the right side, since sometimes that makes it easier to find the determinant.
8b-b-(-12-b^2)=0.  Watch the signs here. We go left to right with 6*0*1=0 + b*-2*-4=8b+1*-b*1=-b.  That is 7b.  Then we subtract the quantity 1*0*4+-2*1*6+-1*b*-b=-12-b^2. That is adding 12+b^2.  Then we have the quadratic equation below
7b+12+b^2=0
b^2+7b+12=0
(b+4)(b+3)=0
b=-4,-3
The matrix is 
6===4===-4
-4==0====1
1===-2===1, and that determinant is 28-28
and
6===3===-4
-3===0===1
1===-2===1, and that determinant is -21-(-21)