You can use the formula ...
... (percent change) = ((new amount) - (original amount))/(original amount) × 100%
Filling in the given information, this tells you ...
... (percent change) = ($21 -$28)/($28) × 100%
... = (-7/28) × 100%
... (percent change) = -25%
A negative percent change is a decrease, so the change in price is a ...
... 25% decrease
