Must you? Yes I must!

This is a portion of an email that I sent to my sister a while ago. She told me recently that she thought it was funny (I thought I was just solving a math problem). Maybe you will think it's funny too.

"The other day I got a phone call from a friend. The last 4 numbers
of her phone number are the same as yours. Even though it was a
Kingston number on the call display, I thought you were calling. Her
name is Larissa, but I heard Melissa. I wrote her the following email
in response, which you might also find amusing.

If x+Larissa=Melissa, what is x?

The last four digits of your phone number are 1234 [1].
The last four digits of my sister's phone number are 1234.
Thus, when I saw your number on my call display, I thought you were my sister.
My sister's name is Melanie.
Therefore, x=Melanie."

[1] Number changed to protect the identity of the phones involved.