Performance index calculation | Statistics homework help

  
Q1
If the USL is 6 and the LSL is 3.5, find the Ppk when the process standard deviation is 1.2
choices are:
a) 0.735
b) 0.264
c) 1.292
d) 0.431 
This question deals with finding the Ppk, which is a performance index that accounts for processes that are not centered. Of the two calculations, we assign the smaller value to PpK. 
Given data are: the upper specification limit (6) and lower specification limit (3.5) for a process, the standard deviation (SD=1.2) of the process. To find the Ppk , we do two calculations:
Of the two calculations, we assign the smaller value to PpK. 
The two calculations are specified as follows:
1. Ppu=(USL- Xbar)/3* SD
2. Ppl =(X bar-LSL)/3*SD
The issue is that I am stuck in finding X bar, which is the mean of the process, to do the calculation. I need help in finding the X bar.
Is there any other formula for this type of problem?

 

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