What does the X mean in the MTBF eauqtion?

Hi, Gurus,
Attachment is one MTBF picture searched in the Chinese webpage.
There is an example in the picture:
Say confidence=90%, and MTBF=300 hours, and failure number is 3,
then
T=0.5*(X0.9(2*3+2))^2*300=524 hours.
Here what confused me is how can they calculated 524 hours?
What does X mean? Integration?
Best,
Tony Liu

Hi,
Now I found it is "chi-square" distribution.
Attachment is one chart for this distribution.
Say 90% probability, and 3 failures as 3 freedom, then it is 0.58,
so T=(0.58)*8*0.5*300=696 hours, how the aforesaid picture calculated 500 hours?
So confused.
Pls comment.
Thanks.
Best,
Tony Liu