Confidence interval for variance

In 16 test runs the gasoline consumption of an experimental engine had a slandered deviation of 2.2 gallons. Find 99% confidence intervals for variance , which measures the true variability of the gasoline consumption of the engine.

Solve it using this rule:

http://sixsigmatutorial.com/Six-Sigma/Six-Sigma-t-Confidence-Interval-Variance.aspx

N=16

S^2=(2.2)^2

I don't know how to calculate x^2 alpha/2,n-1

Answer:

2.21< < 15.78