Since both fractions have the same denominator, you can simply add up (or substract) the numerators and keep the denominator unchanged :
x2+3x+2x2−16+3x+6x2−16=(x2+3x+2)+(3x+6)x2−16
=x2+6x+8x2−16.
Then, you can factorize it :
x2+6x+8x2−16=(x+2)(x+4)(x+4)(x−4)=x+2x−4.
In the case of a substraction :
x2+3x+2x2−16−3x+6x2−16=(x2+3x+2)−(3x+6)x2−16
=x2−4x2−16=(x+2)(x−2)(x+4)(x−4).
