Calculations for field in adobe forms

[livtin]

I am trying to make a field conditional on the content of another field in adobe forms
eg
if F9 is "empty" then return an "empty" field
else return the "result of formula" - keep getting syntax errors and can't figure out where I'm going wrong

I also need to calculate
0.007184* X to the power 0.725*Y to the power 0.425. where x & y are the values of other fields
I've been trying to use Math.Pow(x,y) but it doesn't seem to be working. My formula has been accepted but the field remains blank.

Any assistance greatly appreciated