A glitch in the problem set

In problem 69 (a) you are not asked to calculate the inverse (as I by mistake wrote), only the determinant - now you can do that by cofactor expansion.

For 139 (b) you shall calculate the inverse though. 

(Of course you can also calculate the inverse in 69 (a) as well, by using cofactors, but do not expect that to be given priority in the seminar.)