The question is saying that Donna made 1 call, which cost $0.14 per minute, while using a prepaid phone card with a value of $20. After that 1 call, she had spent $x to be left with $13.84 on the card.
To find how long the call was, you need to know how much she spent.
20 - 13.84 = 6.16
So Donna spent $6.16 on that 1 call.
$6.16 / $0.14 = 44
Donna's call lasted 44 minutes, as we can see, since she spent $6.16 and it costs $0.14 per minute.
