You have gotten advice that you should spend no more than 30% of your take-home pay on rent. Your take-home pay is $1,400 per month. How much can you afford to pay for rent per month?
If you want to find how much a percentage amount represents out of a given number you multiply the two. In our case we have 30/100 (or 30%) * 1400 = 30*1400/100 = 30*14 = 420. So you can afford to pay $420 for rent per month.