What is the sum of 2X plus 1 and its opposite?
Zero. The sum of anything and it's opposite is zero, that's how
an opposite is defined.
In this case, the opposite of 2x + 1 is -(2x + 1) = -2x - 1 by
the distributive property. Adding like terms, 2x + -2x + 1 +-1 = 0
+ 0 = 0.