symmetrical 3-phase electromotive force

Somebody please,how to find this?

Find the sum of the Symmetrical three-phase electromotive force e_a,e_b,e_c.

e_a(t) = E_m.sinwt
e_b(t) = E_m.sin(wt - 2pi/3)
e_c(t) = E_m.sin(wt - 4pi/3)

e_a+e_b+e_c = ?