I$\text{'}$m trying to calculate, in RStduio, the mean growthrate of trees $10$ years old and $15$ years old. I've added a new column called "growthrate" in a previous question.

Here is my code, I don't know if I am even close.

df $<-$ df $\%>\%$ filter$($age $\%$in$\%$ c$(\text{'}10\text{'},\text{'}15\text{'}))\%>\%$

mutate$($averageheight $=$ mean$($Loblolly$height$))\%>\%$

group$\_$by$($Loblolly$age$)\%>\%$ summarise$($averageheight$)$