​ Slope

For 1st month = 80

For 2nd month = 80 + 30

                         =110

For 3rd month = 110 + 30

                        = 140

For 4th month = 140 +30

                       = 170

So, for 4th month is $170

Notice  that it appears that every month, the total cost increases  by  $30

And  the  starting cost  = 80 =   $50 fixed  cost + $30 variable cost

So....letting C   be  the cost in any month and M  be  the  month, we  have  that

C =  50  + 30M

When M  =4, the total cost , C   =   50 + 30 (4)  =  $( 50 + 120)  = $170