If F1 is the magnitude of the force exerted on the Sun by Earth and F2 is the magnitude of the force exerted on Earth by the Sun, then:
F1=F2
According to Newton's law of gravitation, the force exerted by two masses on each other separated by certain distance is equal in magnitude and opposite in direction.
So, the force exerted on the Sun by Earth and the forec exerted on Earth by the Sun will be equal in magnitude and opposite in direction.
∴ F1 = F2