If we work together, we'll probably be able to finish this.
Literal
Strength [object-を] join-ば, this [object-を] finish can [probably-だろう].
力を合わせる ('to join forces, pool efforts, work together') is a common team idiom. ~ば marks the conditional 'if.' 終わらせる is the causative of 終わる ('cause to end' = 'finish something'). ~だろう for prediction.