When a body of mass m slides down on inclined plane with acceleration a, then various forces acting on the body are shown in the figure.
According to Newton's second law of motion, ma=mg‌sin‌θ−µR ⇒‌‌ma=mg‌sin‌θ−µmg‌cos‌θ ⇒‌‌a=g‌sin‌θ−µg‌cos‌θ ⇒‌‌a=g(sin‌θ−µ‌cos‌θ)