Step 1: Analyzing the equation.
The equation given is \( x = at + bt^2 \). The distance \( x \) is in kilometers, and time \( t \) is in seconds.
Step 2: Determine the unit of \( b \).
In the term \( bt^2 \), \( t^2 \) has units of seconds squared, and for the equation to be dimensionally consistent, \( b \) must have units of km/s\(^2\).
Step 3: Conclusion.
The unit of \( b \) is km/s\(^2\).