Question 33642
let us use variables:
B[a b, c d]
A=[1 -2, 1 0]
AB=[1 0, -1 2]
A[1 -2, 1 0]*B[a b, c d]
AB[a+b -2a, c+d -2c]=[1 0, -1 2]
from this, we know that a=0 and c=-1
AB[0+b 0, -1+d 2]=[1 0, -1 2]
from this, we know that b=1 and d=0
So..... B[0 1, -1 0]

I solved by using:
A[a b, c d]
is
................................
I a , b I
I c , d I