Could you please help me with this question? and explain it

For each of the following sets X;, decide whether it is a subspace of the given vector space Uj. If it is, show that it is non-empty and is closed under both addition and scalar multiplication. Otherwise, explain why the set is not a subspace of Ui. (a) U1 = M3(R), X1 = {(ajj)i,j E M3(R) | ajj > a; ; whenever i > j). (That is, X1 consists of those 3 X 3 matrices in which the (i, j) -entry is greater than or equal to the (j, i)-entry whenever i 2 j.) (b ) U2 = F, X2 = {f E F| f (x+ 1)= f(x) for all x ER}