4x + y = 0
-x - 2y =7
with - x - 2y = 7
we have x = -2y - 7
we replace x by this value in the first one
4 * (-2y - 7) + y = 0
- 8y - 28 + y = 0
- 7y = 28
y = 28/(-7)
y = - 4
since 4x + y = 0
we have 4x + (-4) = 0
4x = 4
x = 1
(1 ; -4)
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