Replace the third equation by the sum of itself and -1 times the second equation x-2y-7z=17,-6x+4y+5z=-9,8x+7y=z=-4

Step-by-step explanation:

--- 1

--- 2

---3

-1 times the second equation =

                                                 =  --4

Now we are given that replace the third equation by the sum of itself and -1 times the second equation

So, New third equation =

                                       =

Hence the new third equation is

x=6 and x=3

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