A particle is acted on by two torques about the origin r1 has a magnitude of 2.0 N ∙ m and is directed in the positive direction of the x axis, and r2 has a magnitude of 4.0 N ∙ m and is directed in the negative direction of the y axis. In unit vector notation, find dℓ/dt where ℓ is the angular momentum of the particle about the origin.