Incline with Friction

On a ramp $N = mg\cos\theta$, so kinetic friction is $f_k = \mu_k mg\cos\theta$, along the slope against the sliding. Sliding down, with down the slope positive:

$$mg\sin\theta - \mu_k mg\cos\theta = ma$$

Sliding down: friction points up the slope, against the sliding.

A box moving up the slope feels friction down the slope, on the same side as gravity's pull.