3x+7=-x-1
move -x to the other side
sign changes from -x to +x
3x+x+7=-x+x-1
3x+x+7=-1
4x+7=-1
move 7 to the other side
sign changes from +7 to -7
4x+7-7=-1-7
4x=-8
Divide by 4 for both sides
4x/4=-8/4
Answer: x=-2
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