The question is:

Let A= [2 5, -3 1] and B=[4 -5, 3 k]

I don't know if this will represent A and B better:

[2 , 5] [4 , -5]

[-3 , 1] [3 , k]

What value(s) of k, if any, will make AB=BA?

I got no values of k, but the back of my book claims the answer is (k-5). Can anyone tell me if I am right or wrong?

Thanks,

jarny