Combine two or more columns in a dataframe into a new column with a new name

For example if I have this:

n = c(2, 3, 5) 
s = c("aa", "bb", "cc") 
b = c(TRUE, FALSE, TRUE) 
df = data.frame(n, s, b)

  n  s     b
1 2 aa  TRUE
2 3 bb FALSE
3 5 cc  TRUE

Then how do I combine the two columns n and s into a new column named x such that it looks like this:

  n  s     b     x
1 2 aa  TRUE  2 aa
2 3 bb FALSE  3 bb
3 5 cc  TRUE  5 cc