Attached a diagram of the scheme described in the problem ..
To solve the problem we need to know the relationship between kinetic and normal force, so
[tex]F_k=\mu_k F_N (1)[/tex]
Where
[tex]\mu_k =[/tex] coefficient of kinetic
[tex]F_N=[/tex] Normal force
We perform the sum of forces as well,
[tex]\sum F_x = Ma[/tex](2)
[tex]a= Acceletarion[/tex]
For Normal Force in Y,
[tex]F_N= Mgcos \theta[/tex] (3)
The force in X,
[tex]\sum F_x = Mgsin\theta - F_x[/tex] (4)
Replacing in (4)
[tex]Ma = Mgsin\theta - \mu_k F_N[/tex]
[tex]Ma = Mgsin\theta - \mu_k F_N[/tex]
[tex]Ma = Mgsin\theta - \mu_k Mg cos\theta[/tex]
[tex]a= g(sin\theta-\mu_k cos\theta)[/tex]
In this way, it does not matter which object is chosen.