Well, observe that for any scalar and nxn matrix A.
where I is the standard nxn identity matrix. So you got . So you know the trick for determinats when multiplying matricies, Det(AB) = Det(A)Det(B). Well, the determinant of any matrix with entries only in the diagonals and zeroes everywhere is just the whole diagonal multiplied, since there are n diagonal enteries (all of them m),
so you got