You have the following algebraic expression:
+4x² - 4x²
In order to simplfy the previous expression you take into account that you can sum or subtract terms with variables with same exponents. In this case you have two term with x², then, you can subtract them. To do that you simply subtract the coefficients of the terms:
+4x² - 4x² = 0
The answer is zero, because the terms have the same coefficients but opposites sings.
