There is a downward force of m*g on the block due to the gravitational attraction of the Earth. A component equal to `m*g*cos theta` is perpendicular to the incline. This is the normal force of the block on the incline. The resulting force of friction is `mu*m*g*cos theta` . This gives the net force of the block along the incline as `m*g*sin theta - mu*m*g*cos theta`

The acceleration of the block due to this force is `g*sin theta - mu*g*cos theta`