You simplify an expression by combining all like terms.

For example, given the expression 3x^2 - 2 + 5x + 3 - x^2 + 2x + 13,

you could first combine all the constant terms:

3x^2 - **2** + 5x + **3** - x^2 + 2x + **13**

3x^2 + 5x - x^2 + 2x** - 2 + 3 + 13**

3x^2 + 5x - x^2 + 2x **+ 14**

Then you could combine the x terms:

3x^2 + **5x** - x^2 + **2x** + 14

3x^2 - x^2 + **5x + 2x** + 14

3x^2 - x^2 **+ 7x** + 14

And finally the x^2 terms:

**3x^2** - **x^2** + 7x + 14

2x^2 + 7x + 14

2x^2 + 7x + 14 is the simplified expression of 3x^2 - 2 + 5x + 3 - x^2 + 2x + 13.