If and only if it is a proportional relationship, the problem can be solved by creating a proportion of B1/T1=B2/T2. This can also be expressed as 4.5/3.5=5.4/x given the values. You must cross multiply, so 4.5x=3.5(5.4). This simplifies to x=4.2. So, the tail length would be 4.2 feet.
Hope this helps.