Answer
$$x^4-b^2x^2-a^2x^2+a^2b^2$$
Work Step by Step
Multiplying the expression out, we find:
$$\left(x+a\right)\left(x-b\right)\left(x-a\right)\left(x+b\right) \\ \left(x^2-bx+ax-ab\right)\left(x-a\right)\left(x+b\right) \\ \left(x^3-bx^2-a^2x+a^2b\right)\left(x+b\right) \\ x^4-b^2x^2-a^2x^2+a^2b^2$$
