Is ?The set M(2,2) of a 2x2 matrix, with real entries, is a vector space.

The set of diagonal matrices

D=(a 0 )

( 0 b )

where a, b are real numbers. is a subset of M(2,2).

i)

Write down two particular matrices which belong to D, and two particular matrices which belong to M(2,2) but not D.

Two matrices which belong to D are

(a) (0)

v =(0) u=(b)

I don't get the other part.. which does not belong to D.