A 248-kg crate rests on a surface that is inclined above the horizontal at an angle of 19.5°. A horizontal force (magnitude = 537 N and parallel to the ground, not the incline) is required to start the crate moving down the incline. What is the coefficient of static friction between the crate and the incline?
can someone please help me?

Can you find the net force on the crate down the plane (adding component due to gravity, friction and the component of the force applied)?

The sum of those forces should be zero, since the applied force is enough for the crate to 'start' moving.