Why we Dropped (das) article in the sentence? ich möchte für nächstes wochenende ein zimmer reservieren​.​

the answer is simple; we dropped (das) and the added (ein) because we were not referring to something special and there's no certainty for it.

for example, in English, we say: I like to reserve a room for next week. ( Note that we are not looking for a special room and we just want a room).

answer:

seven. five plus one is six. five + 1+1 is 7. 1+1 equals two. so just add 2 to five. seven.