のではないだろうか is a fairly long grammar pattern in Japanese that is used when one wants to express their opinion indirectly, in the form of a rhetorical question. Common translations include ‘I think it’s (A), isn’t it?’, and ‘it might be that (A)’. However, being that this is just a combination of の, ではない ‘isn’t’, だろう ‘probably’, and か, the literal meaning is simply ‘is it probably not that it’s (A)’. Just like in English, the elongation of the phrase and not saying anything with certainty makes it sound more modest.
のではないだろうか may be seen following verbs and い-Adjectives, or な-Adjectives and nouns when preceded with な. Additionally, でしょう may replace だろう.

• 先輩（せんぱい）は疲（つか）れているのじゃないだろうか。
  I think that our senpai is tired, don’t you think?
• 息子（むすこ）がおもちゃをじーっとみている。あのおもちゃが欲（ほ）しいのじゃないだろうか。
  Our son is staring at a toy. I think he wants that toy, don’t you agree?
• 長谷川（はせがわ）さんは田中（たなか）さんのことが好（す）きなのではないでしょうか。
  I think that Hasegawa-san likes Tanaka-san, don’t you agree?
• あの人（ひと）の右腕（みぎうで）に入（はい）ってるタトゥー、防犯（ぼうはん）カメラに映（うつ）っていたのと全（まった）く同（おな）じです。あの人（ひと）は犯人（はんにん）なのではないでしょうか。
  The tattoo on that person's right arm is exactly the same as the one seen on the security camera. I think that he is the perpetrator, don’t you agree?

This phrase can also be used when seeking agreement from the listener, or when mildly trying to persuade them that your way of thinking may be correct.