#### Answer

$$-3x^{2}-4xa-a^{2}$$

#### Work Step by Step

Initial: $x^{2}-(2x+a)^{2}$ First, rewrite the $(2x+a)^{2}$ in its expanded form: $(2x+a)(2x+a)$ Then FOIL it:
$$x^{2}-(4x^{2}+4xa+a^{2})$$ After that, distribute the negative:
$$x^{2}-4x^{2}-4xa-a^{2}$$ Then simplify by combining like terms:
$$-3x^{2}-4xa-a^{2}$$