Final answer:
The affirmative formal command for 'comprar' in Spanish is 'compre'. It is formed by taking the first person singular indicative present tense form, dropping the 'o', and adding 'e'.
Step-by-step explanation:
The affirmative formal command (Ud.) of the verb comprar (to buy) is compre. To form this command, you take the yo form of the present indicative, drop the 'o', and add the opposite vowel ending 'e' for -ar verbs. In this case, 'compro' (I buy) becomes 'compre' (you buy, formal).
The affirmative formal command (ud) of comprar is compre.
For example, to tell someone in a formal setting to buy something, you can say ¡Compre la camisa! (Buy the shirt!)