$\overrightarrow P + \overrightarrow N + \overrightarrow {{F_{ms}}} = m\overrightarrow a $
Ox: $P.\sin \alpha - {F_{ms}} = ma$
Oy: $N - P\cos \alpha = 0$
suy ra: $mg.\sin \alpha - \mu .mg.cos\alpha = ma$
$ \to a = g.\sin \alpha - \mu .g.cos\alpha = 7,22m/{s^2}$