Learn Swedish: Kompromissen blev en bra gemensam lösning. - The compromise became a good common solution.

While var simply means "was", Swedes frequently use blev (the past tense of att bli) to describe the final outcome of a process. Since a compromise is the result of a negotiation, blev perfectly captures the feeling of "turned out to be" or "ended up as".

Why doesn't the adjective bra have a special ending here to match the noun?

The adjective bra is a highly common exception in Swedish. It is completely indeclinable, meaning it never changes form. Whether the noun is an en-word, an ett-word, or plural, it always remains bra.

How does the adjective agreement work with gemensam in this sentence?

Gemensam means "common" or "mutual". Because it describes en lösning (an en-word), it stays in its basic form. If it were describing an ett-word, it would become gemensamt, and for plurals, it would be gemensamma.

How is the word lösning formed?

It is built from the verb att lösa (to solve). The -ning suffix is a very common way to turn a verb into a noun in Swedish, similar to "-tion" or "-ing" in English.