A cell phone regularly sells for $210 is on sale for 30% off.

Several ways to attack this problem. If you got a 30% discount that means you paid 70% of the original price. Of is the key word, it means multiply. 70% of the original amount is $210 so that is your equation:

0.70 * x = 210

x = 300 <-- this is your original price and if you think about it taking a 30% discount of this would be 90 dollars and 300 - 90 = 210

All the best