Answer:
a) t = Â 2.7638
b) t = - 2.6245
c) t = 3.1058    on the right side   and
  t  = -3.1058   on the left
Step-by-step explanation:
a)Determine critical value for a right-tail test  for α = 0.01 level of significance and 10 degrees of fredom
From t-student table we find:
gl  =  10  and α = 0.01    ⇒  t =  2.7638
b)Determine  critical value for a left-tail test  for α = 0.01 level of significance  and sample size n = 15
From t-student table we find:
gl  =  14  and α = 0.01      gl  =  n - 1    gl  = 15 - 1   gl = 14
t = - 2.6245
c) Determine critical value for a two tails-test  for α = 0.01 level of  significance the  α/2  =  0.005  and sample size  n = 12
Then
gl  =  11  and α = 0.005
t = 3.1058 Â Â Â on the right side of the curve and by symmetry
t = - 3.1058 Â
From t-student table we find: