The forces acting on the block are : Normal reaction of the plane perpendicular to the surface, weight of the block vertically down wards. Force of friction acting along the surface upwards. Look at the free body diagram showing forces.
If Ф is the angle of plane = 30 deg, the block just starts sliding down. It means that the force of friction upwards the plane is equal to the component of gravitational force downwards the plane.
mg sin 30 = Mu N and mg cos 30 = N
Hence mg sin 30 = Mu mg cos 30
we get Mu = tan 30 = 0.5773
Now when the angle is increased to 45 degrees, block accelerates with 'a' meters/sec² down the plane.
mg sin 45 - Mu N = m a
Also, mg cos 45 = N. Substitute value of N in the above equation.
So we get mg sin 45 - Mu mg cos 45 = m a
=> a = 9.8 /√2 [ 1 - Mu ] = 9.8/√2 * 0.4227 = 2.929 m/sec²