.
Let the numbers be x, y and z.
Then the problem says
x + y + z = 1 (1)
x - y + z = -1 (2)
x + y - z = -9 (3)
Do not be afraid. Simply subtract equation (2) from equation (1). The terms "x" and "z" will cancel each other and you will get
2y = 1 - (-1) = 2 =====> y = 2/2 = 1.
Next subtract equation (3) from equation (1). The terms "x" and "y" will cancel each other and you will get
2z = 1 - (-9) = 10 =====> z = 10/2 = 5.
As the last step, substitute y= 1 and z= 5 into equation (1). You will get
x + 1 + 5 = 1 =====> x = -5.
Answer. x= -5; y= 1; z= 5.
Solved.
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