Question

substituting x=3 equation x-y=-5

Answer 1

$x - y = - 5 \\ substituting \: x = 3 \\ 3 - y = - 5 \\ - y = - 5 - 3 \\ - y = - 8 \\ y = 8$

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Answer 2

x=3
x-y=5
put the value of x
3-y=5
-y=5-(-3)
y=8
both side minus deducted