You borrowed 90 dollars from your grandmother. you pay back 15 dollars each week. write in standard form an equation in which y is the amount of money owed after x weeks

Okay, so, if you owe 90 dollars, and you pay 15 every week, you need a variable for how many weeks. We're gonna use x. You need another variable to represent how much more money you owe after x weeks. We're using y for this one.
If you're starting amount is 90, we're going to subtract a number from it. To find how many weeks you're paying, use the variable x. If it's 15 dollars a week, we use 15x
90-15x = y