Answer:
You need to rent 14 videos, and you'll pay the same amount for both. Renting 15 will make the membership a better method of payment.
Step-by-step explanation:
Let Number of videos to be rented be x;
Given:
For non members;
Cost of renting 1 video = $4
Cost for renting x videos = 
For Members;
Membership cost = $21
Cost of renting 1 video = $2.50
Cost of renting x video = 
We can create an equation. Say the number of videos you rent is given by x
. We can write that, if we rent x videos without membership, we'll have to pay
. If we rent the same amount of videos with membership, we'll have to pay
.
Hence the equation can written as

Hence You need to rent 14 videos, and you'll pay the same amount for both. Renting 15 will make the membership a better method of payment.