Beginner Gardening: Need an ID for this plant, please, 1 by Kizmo

Kizmo wrote:
This plant was left by a previous tenant, who attempted to kill everything she couldn't take with her by pouring salt over them. We thought it was dead, but it came up with a few sprigs and leaves last summer, and then this year has tripled its size. At first I thought maybe it was a type of spirea, but can't match up both the bloom and the leaves to a picture. I know I've seen it somewhere, but can't place a name to it.
Any help would be appreciated!

Thanks,
Kathy