Assuming Trump bombs Syria, which appears to be likely, what will/can Russia do?
Plenty. What's more, they seem willing to respond.

‘Trump: Get ready Russia, missiles will be coming at Syria, nice and new and ‘smart’!’  via RT, 11 Apr, 2018 11:06

‘Moscow to Trump: Are ‘smart’ missiles an attempt to destroy alleged chem attack evidence in Syria?’, also via RT, 11 Apr, 2018

This post will be quite short because instead of writing, I prefer the readers to watch this Blackstone Intelligence video (no relation to Blackwater). 24 minutes

Brief synopsis:

1. Russia urges 24 hours be allotted for international experts in chemical warfare be allowed to examine Douma

2. Trump marshaling large naval taskforce off coast of Syria, some ships within 100 km of Russian base.

3. France eager to get in on the action

All the cool and popular kids are bombing Syria these days, so the less cool kids want to be involved as well.
France hasn't bombed Syria since it was cool more than two generations ago. They are teaming up with the less than cool Saudis.

CIA Director Mike Pompeo, the same asshat who hammered Hillary Clinton over "Benghazi e-mail", called Clinton for advice in preparing for the Senate confirmation hearings for the cabinet post to head the State Department.

Lost amidst all the hoopla surrounding the false flag chemical attack in Douma, Syria, is the little tidbit that Syrian air defenses are increasingly effective.

Interfax cited the Russian Defence Ministry as saying the Israeli warplanes had carried out the strikes from Lebanese air space, and that Syrian air defence systems had shot down five of eight missiles fired.