jika x1 dan y1 adalah penyelesaian sistem persamaan linear2x-5y-25=0 dan 3x+2y-9=0 nilai x1 - 2y1 =

2x - 5y - 25 = 0 ---> 2x - 5y = 25 .....(i)
3x + 2y - 9 = 0 ---> 3x + 2y =  9 .......(ii)
eliminasi y persamaan (i) dan (ii)
2x - 5y = 25  | *2 |     4x - 10y = 50
3x + 2y = 9   | * 5 | 15x + 10y = 45
                             -------------------- +
                                       19x  = 95
                                         x = 95/ 19
                                         x = 5
subtitusi x = 5 ke persamaan (i)
        2x - 5y = 25
<--> 2*5 - 5y = 25
        10 - 5y = 25
               5y = 25 -10
               5y = 15
                 y = 15/5
                y  = 3
jadi x1 = 5 dan y1 = 3
maka nilai x1 - 2y1 = 5 - 2*3
                               = 5 - 6
                               = -1

jadi nilai x1 - 2y1 = -1