Answer
The interaction partner is the contact force exerted on the floor by the chest. This force has a magnitude of 870 N and it is directed at an angle of $59^{\circ}$ below the rightward horizontal.
Work Step by Step
The floor exerts a contact force on the chest. This contact force has a magnitude of 870 N and it is directed at an angle of $59^{\circ}$ above the leftward horizontal.
By Newton's third law, there is an interaction partner of this force. The interaction partner has the same magnitude and it is directed in the opposite direction.
The interaction partner is the contact force exerted on the floor by the chest. This force has a magnitude of 870 N and it is directed at an angle of $59^{\circ}$ below the rightward horizontal.
