Please see attached mathematical statistics question and additional info below. If n = 13 and the significance level alpha = 0.025, how to determine c?

4.2.18. Let X1, X2, . .., Xn be a random sample from N(#, o?), where both parame- ters / and o' are unknown. A confidence interval for o can be found as follows. We know that (n - 1)$2/o' is a random variable with a x2(n - 1) distribution. Thus we can find constants a and b so that P((n - 1)$2/02 02 > > (n-1)$2/b. Hence the second probability statement can be written as P((n - 1)$2 /b