# 1)
df <- 12
# P[Y <= -2]
pt(-2, df)
# P[Y >= 3]
pt(3, df, lower.tail = FALSE)
# p[-2.6 =< Y <= 2.6]
1-2*pt(-2.6, df)
# 2)
qt(0.065, df)
# 3)
qt(0.91, df, lower.tail = FALSE)