Step 1 - FBD: Draw a free body diagram of the crate.
Step 2 - Kinetics: Write down the Newton/Euler equations for the crate. Be careful with your moment equation. Since the crate has no fixed points, you need to sum moments about the center of mass of the crate.
Step 3 - Kinematics: Crate does not tip, so α = 0.
Step 4 - Solve