1 sampe test for trees aged 15 in loblolly in R data base

I'm trying to Run a 1-sample t-test for trees aged 15 to see if their mean height is 34 feet. Use the function t.test() and be sure to set the argument alternative="two.sided". You can use alpha=0.05.

I'm trying to define trees aged 15 as a subset but have been unsuccessful. I've tried:

Loblolly2 <- Loblolly[Loblolly$age != 3,15,20,25, ]

Any ideas?