Hoping to get some help with questions that Im stuck on for exam revision

Show that:

a) U = (x,y,z) x+y=z is a subspace of R3

b) U= (x,y,z), y is rational and is not a subspace of R3

c) U= A|A2=A,AϵMmn is not a subspace of Mmn

d)U={F|f(x+pi)=f(x), FϵC(-infinity, + infinity) is a subspace of C(-inifity, +inifinity)

e) u= p(x)|p(1)=0, p(x)ϵPn is a subspace of Pn

Cheers