Question 1194003
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The bottom row of 0, 1, -1 tells us we have the equation 0x + 1y = -1
That leads to y = -1


The top row of 1, 3, 6 tells us we have the equation 1x+3y = 6 or simply x+3y = 6
Plug in y = -1 and solve for x. 
This is the back-substitution portion.
Substitution because we replace y with -1
The "back" part refers to us going back up the matrix.


x+3y = 6
x+3(-1) = 6
x-3 = 6
x-3+3 = 6+3
x = 9


The solution is (x,y) = (9, -1)
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