x + x2 + 2x2 - 3x

Put a 1 before each of the first two terms to show their understood
coefficient:

1x + 1x2 + 2x2 - 3x

The two red terms are like terms.
The two blue terms are like terms.

Combine the two red terms 1x - 3x = -2x

Combine the two blue terms + 1x2 + 2x2 = 3x2

Answer:  -2x + 3x2

If you want it in descending order, then reverse the terms:

+ 3x2 - 2x

Now you can eliminate the + in front:

3x2 - 2x

Edwin