You can do it a couple of ways. The easiest is to just take out the row you don't want, the 18th row. It's the same as taking 18
out of the sequence from 1 to 21.
> c(1:21)[-18]
## [1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 19 20 21
So on the data frame, you can remove the 18th row with original_data_frame[-18, ]
.
Or, you can do it the other way by taking the rows you do want
> c(1:17, 19:21)
## [1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 19 20 21
which would be original_data_frame[c(1:17, 19:21), ]