(a). The acceleration of the carton is found by calculating the net force on the carton divided by its mass, ∑F/m. There are four forces, two of which cancel each other out.
Fpar = W sin θ = mg sin θ. [force parallel to the incline.]
Fperp = W cos θ = mg cos θ. [force perpendicular to the incline.]
Fnor = Fperp [normal force, opposite in direction to Fperp.]
Ffric = µk * Fnor [force due to friction, opposite in direction to Fpar.]
a = ∑F/m = (Fpar - Ffric)/m = (mg sin θ - µk * mg cos θ)/m = g sin θ - µk * g cos θ = g(sin θ - µk * cos θ).
a = (9.81)(sin 24.5 - 0.07*cos 24.5) = 3.443 m/s2.
(b). Try this one considering the acceleration has been calculated in (a).