Used to form a rhetorical question or strong assertion ('isn't it...?'), expressing the speaker's expectation or confirmation.
このやり方の方がいいのではないか。
Wouldn't this way be better?
彼の説明は少し変ではないかと思う。
I think his explanation is a bit strange, don't you?
早めに出発したほうが安全なのではないか。
Wouldn't it be safer to leave earlier?
Followed by the volitional form of a verb to make a suggestion or rallying call ('let's...'; 'shall we...?'). This usage is somewhat formal or literary.
困っている人がいるなら、助けようではないか。
If someone is in trouble, let's help them.
最後まで頑張ろうではないか！
Let's do our best until the end!