Bracket notation is a way to get a character at a specific index within a string.
Most modern programming languages, like JavaScript, don't start counting at 1 like humans do. They start at 0. This is referred to as Zero-based indexing.
For example, the character at index 0 in the word "Charles" is "C". So if var firstName = "Charles", you can get the value of the first letter of the string by using firstName[0].
Use bracket notation to find the first character in the lastName variable and assign it to firstLetterOfLastName.
Hint
Try looking at the firstLetterOfFirstName variable declaration if you get stuck.