Draw a free-body diagram showing the three forces acting on the box: weight straight down, normal force perpendicular to the incline up, and friction along the incline opposite to the direction of motion.
Call the direction parallel to the incline the x-direction and the direction perpendicular to the incline the y-direction. Find the sum of all forces in the x- and y-direction:
∑Fx=mg sin(25) - Ff
Use Newton's 2nd law for each: in the x-direction, ∑Fx=ma, while in the y-direction ∑Fy=0.
mg sin(25) - Ff = ma
Fn- mg cos(25) =0
Get the normal force: Fn = mg cos(25)
Then the force of kinetic friction is: Ff = µ Fn = µ mg cos(25)
mg sin(25) - µ mg cos(25) = m a
Divide through by m, the answer is independent of the mass:
a = g sin(25) - µg cos(25)
a = 9.8 m/s² ( sin(25) - 0.15 cos(25) )
a = 2.8 m/s²