Any help is much appreciated :)

Can someone me out here and the tell me the greatest common factor

Having a hard time understanding this question (see attachment).

The value, v, of kalani’s stock investments over a time period, x, can be determined using the equation v=750(0.80)^-x. what is the rate of increase or decrease associated with this account? 20% decrease 20% increase 25% decrease 25% increase