Bad Drywall seam

I recently removed wall paper and after a few hot water baths and washing the glue off. I painted the wall. After the paint had dried, it revealed two bad horizontal seams in one of the stairwell walls. The seams are slightly rounded to the point that it is visible in any light. What is the best way to repair this? Mine out the seam, sand down the edges and repair it with tape and mud??

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