う・よう + ではないかLet's do something, Why don't we

ようではないか is a construction that will primarily be used for making suggestions for mutual activities. This is a combination of either of the auxiliary verbs う or よう, ではない, and the sentence-ending particle, か. う and よう primarily express will or intent like 'shall' in English. Due to the negation following them, the literal translation will be similar to 'shall we not do (A)?'. For ease of translation, this structure can be remembered as simply 'let's do (A)', or 'why don't we do (A)'.

As with all structures that use う or よう, ようではないか will be attached to the end of a verb indicating the action which is being suggested.

Alternatively, the abbreviated form of じゃない may be used in place of ではない.

Despite this, ようではないか is primarily a formal grammar structure, so will most often use the unabbreviated version, or even more polite ようではありませんか construction.

Fun-fact

Occasionally, this grammar structure may be seen preceded by もらう. In these cases, it is usually challenging the listener to do something. This can be interpreted as 'why don't I have you do (A) then'.