Forward Propagation, Backward Propagation.
z^l, maybe no surprise there. Dw^l becomes 1 over m, dz^l times a^l minus 1 transpose. Then db^l becomes 1 over m np.sum dz^l. Then axis equals 1, keepdims equals true. We talked about the use of np.sum in the previous week, to compute db. Then finally, da^l minus 1 is w^l transpose times dz of l.