How to do it...

1. Let's declare the basic implementation for the function:

fun calculateDisplacement(initialSpeed: Float, 
                          acceleration: Float, 
                          duration: Long): Double =
    initialSpeed * duration + 0.5 * acceleration * duration * 
    duration

2. Let's declare a default value for the initialSpeed parameter:

fun calculateDisplacement(initialSpeed: Float = 0f, 
                          acceleration: Float, 

                          duration: Long): Double =
    initialSpeed * duration + 0.5 * acceleration * duration * 
    duration