To find out how many books Mateo's teacher sent him, we can set up the equation as follows:
Let \( x \) represent the number of books his teacher sent. Initially, Mateo has 340 books, and after receiving more books, he has 352 books.
The equation can be expressed as:
\[ 340 + x = 352 \]
This equation states that the initial number of books (340) plus the number of books received (x) equals the total number of books after receiving them (352).
So the correct equation is:
340 + x = 352