Inverse Proportion: Brightness and Distance Squared

The brightness of an object, \(b\) \(\mathrm{W}/\mathrm{m}^2\) with respect to a light source is inversely proportional to the square of the distance, \(d\) metres, from the light source.

1. Write down a formula connecting \(b,d\) and a constant \(k\).
2. When the object is \(5\) km from the light source, the brightness is \(2\) W/m\(^{2}\). Find the brightness when the object is \(10\) km from the light source.
3. When the object is a certain distance from the light source, the brightness is \(x\) W/m\(^{2}\). What is the brightness in terms of \(x\) when this distance is halved?