Hello!
Since Linda has 871.43 and the subtract “x” money every month the equation is... 0=871.43-x*y where “x” is the monthly fee and y is the number of months that have passed. If x=9 meaning the monthly fee is 9 dollars a month, then in the equation you put x=9 to get 0=871.43-9y and solve for y, number of months.
9y=871.43
y=96.83 or about 97 months.
I hope I could help! :)