\[(x + 1)^{2} - (x - 1)^{2} =\]
\[= \left( x + 1 - (x - 1) \right)\left( x + 1 + (x - 1) \right) =\]
\[= (x + 1 - x + 1)(x + 1 + x - 1) =\]
\[= 2 \cdot 2x = 4x.\]