In: Physics

Gravitational acceleration = g = 9.81 m/s2
Mass of the box = m = 3 kg
Angle of the ramp = 
Normal force on the box from the ramp = N
Coefficient of friction =
= 0.3
Friction force on the box = f
f =
N
The box is moving at constant velocity therefore the acceleration of the box is zero that means all the forces on the box are balanced.
From the free body diagram,
N = mgCos
f =
mgCos
f = mgSin
mgCos
=
mgSin
= Tan
Tan
= 0.3
=
16.7o
Angle of the ramp that will cause the box to slide at constant velocity = 16.7o