When I do problems involving real roots, such as x = 1, why is it always x = 1 or x = - 1?

For example:

Determine the real roots of each equation:

$\displaystyle (5x^2+20)(3x^2-48)=0$

the real root is x = 4 or x = - 4. Why is it always +/- or how can I identify this issue?