(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.
F_{par} = W sin θ = mg sin θ. [force parallel to the incline.]
F_{perp} = W cos θ = mg cos θ. [force perpendicular to the incline.]
F_{nor} = F_{perp} [normal force, opposite in direction to F_{perp}.]
F_{fric} = µk * F_{nor} [force due to friction, opposite in direction to F_{par}.]
a = ∑F/m = (F_{par}  F_{fric})/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/s^{2}.
(b). Try this one considering the acceleration has been calculated in (a).
11/23/2012

Robert B.
Comments
Thanks I got it :)