If you would like to find the solution of the system of equations, you can do this using the following steps:
x - 8y = 21 /*(-3)
3x - 8y = 15
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-3x + 24y = -63
3x - 8y = 15
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-3x + 24y + 3x - 8y = -63 + 15
16y = -48 /16
y = -48/16
y = -3
x - 8y = 21
x = 21 + 8y
x = 21 + 8 * (-3)
x = 21 - 24
x = -3
(x, y) = (-3, -3)
The correct result would be (-3, -3).