So if the discount is 20%, then 80% of the original price is left as the discounted price. (already stated in question)
So what you want to do, is take 80%, or 4/5, and take x as well. Set x equal to the original store price, and as we'll be taking 20% off, it'll become 4/5x, or 80% of x. Set 4/5x equal to 16.8.
Now, to solve, just divide both sides by 4/5 to isolate the x. 4/5x / (4/5) = 16.8 / (4/5) I would set the divided by 4/5 part on the right hand side of the equation into multiplied by 5/4, as any division by a fraction can be set to multiplying by that fraction's reciprocal.
x = 16.8 • 5/4
Cancel out the denominator of 5/4 by dividing 16.8 by 4: x = 4.2 • 5
Now multiply:
x = 21
You get your answer, of the original price being $21.