What is the affirmative usted command of comer?

The affirmative usted, generally, takes the Yo- indicative form of a verb, and uses the opposite ending of what would be normal for that verb. (for er/ir verbs, the usted command will end in "a", for ar verbs, the usted command will end in "e"). This form is very similar, and usually identical, to the present subjunctive. 

Our verb is comer:
Since Comer is a regular verb, the yo form is like any indicative form, "com"
We add the opposite of the typical ending, which means we use "a"

Affirmative Usted Command:
Coma

Example:
"Coma esa mansana, para que no tenga que ir al doctor"