Firstly,

add the following two lines to the definitions area:

(define str "helloworld")

(define i 5)

create an expression using string primitives that adds "_" at position i. in general this means the resulting string is longer than the original one; here the expected result is "hello_world".

position means i characters from the left of the string—but computer scientists start counting at 0. thus, the 5th letter in this example is "w", because the 0th letter is "h".

secondly,

use the same setup, create an expression that deletes the ith position from str. clearly this expression creates a shorter string than the given one; contemplate which values you may choose for i.