When a body is placed on a rough plane (coefficient of friction = μ ) inclined at an angle θ to…

Here is the diagram of the given situation,

Friction will be acting in the direction opposite to the tendency of relative motion, $f=\mu N=\mu mg\cos \theta$

Writing the force balance equations, 

$$\begin{gathered} mg\sin \theta -f=ma \\ \Rightarrow mg\sin \theta -\mu mg\cos \theta =ma \\ \Rightarrow a=g\sin \theta -\mu g\cos \theta \\ \Rightarrow a=g\left(\sin \theta -\mu \cos \theta \right) \end{gathered}$$