How to overwrite position value with variable

Is it possible to overwrite a specific value with a value of a variable from my expression?

My expression (I thought I could use):

"z = transform.position[2]-thisComp.layer("Audioamplitude").effect("Beide Kanäle")("Schieberegler");

transform.position[2] = z;"

But AE claims that the Z-Position Value of my layer (camera) is read-only...

I want to have jumps in my camera synced to musik so the amplitude value should be substracted from my Z-position value but if I dont overwrite the "old" value then the camera obviously jumps back to the original position.

Thanks in advance for any answer