You have a linear function with the values of

     = 16.64  at  x = 28,   and

     = 12.20   at  x = 65.


Hence, the slope of the straight line is

    m =  =  = -0.12.


    // Surely, you do understand that  |m| = |-0.12| = 0.12 dollars is the cost rate (= price) for one minute of the phone conversation.



Now the value of  "y" at x= 89 minutes is

    y =  +  = 12.20 -0.12*(89-65) = 9.32 dollars.


Answer.  The remaining credit after 89 minutes of calls is 9.32 dollars.