I don't get it...even after solving it.

Its simple math. If you uncommented both lines (which is not the solution) here is what the results of each lines would be:

int x = 2; // declares 'x' and sets it to 2
int y = 12; // declaras 'y' and sets it to 12

y = x * y; // sets y to 2 * 12 (y is now 24)
y = x + y; // sets y to 2 + 24 (y is now 26)

x = y - x; // sets x to 26 - 2 (x is now 24)
y = y - x; // sets y to 26 - 24 (y is now 2)

System.out.println(x); // prints 24
System.out.println(y); // prints 2

As a programmer you need to be able explain what each line in a code does. Most college programming courses mandate that you comment each line of code as a way to demonstrate your understanding. While this is not a college course, I recommend that you practice doing the same. If you are able to explain what you want to do in comments, writing code becomes a lot easier. Watch the first video on this forum post, Its a day one lecture for a programming class that has a lot of great tips and explains how to succeed and what to expect through learning to code.