The total sales made by a salesperson was $25,000 after 3 months and $68,000 after 23 months. predict the total sales after 41 months.

You make each into an ordered pair so you will get (3,25,000) and (23,68,000)
So then you will plug it into the slope formula of y2-y1
 68,000-25,000        43,000                               
=     = 2150                    x2-x1                                   
         23-3                     20
 now you plug it in 
y=2150x+b
25,000=2150(3)+b
25,000=6450+b
-6,450  -6,450

18,550=b
put everything together 
y=2150x+18,550
y=2150(41)+18,550
y=88,150+18,550
y=106,700  dollars for 41 months