What is the Correct Z-Transform of x(n) = -2^n u(-n-1)?

[nelectrode]

Hi guys,

I am trying to find the z-tranform of the following equation: x(n) =-2ⁿ u(-n-1)

Using the Z-transform definition,summation and geometric series
I am getting 1/(1-2z^-1)

But according to my lecturer the answer is suppose to be 0.5/(1-2z^-1)

I have tried simplification/factorization and still seems to be wrong.

Thanks

 

[mfb]

What does u mean?
Can you show your steps?