It would be easier to help you, if you provide some sample of your dataframe (an image of it is not easy to work with), so could you please ask this with a minimal REPRoducible EXample (reprex)? A reprex makes it much easier for others to understand your issue and figure out how to help.
This is an example making some assumptions about the structure of your actual data
df <- data.frame(stringsAsFactors = FALSE,
x = c("1.41\n(1.19, 1.66)", "0.10\n(0.05, 0.15)"))
library(tidyr)
library(stringr)
library(dplyr)
df %>%
mutate(x = str_remove_all(x, "[()]")) %>%
separate(x, into = c("mean", "low", "high"), sep = "\n|,", convert = TRUE)
#> mean low high
#> 1 1.41 1.19 1.66
#> 2 0.10 0.05 0.15