I hadn't heard of King's Property but a quick Google says that it is
You prove this using integration by substitution. The version that you use for all of these questions is:
For the first one, let I equal to the value of the definite integral. By King's Property, you can say that
You can then add two of I together to get:
For the second one, you apply King's property to show that
If J is the value of the definite integral, then you can say that
For the last one, it's very similar so I'll skip some steps. If K is the value of the definite integral, then