Determine: The r.m.s. ripple voltage

Here is my attempt.

Vc= Vs-2Vd

=28-2(0.7)

= 26.6 Volts

Vr= (1/2 x fs x C x Rl )(Vc)

R=V/I 28/20= 1.4

(1/2x400x1.4)(26.6)

= 0.023

Vrms= Vr/2sqrt3

Vrms = 0.0063

Now theres a good chance I'm way of here.....Thanks for any help!