Shower steam math

If Fiona's the first one up she usually comes for a cuddle in bed with me. When I get up for a shower she usually likes to follow me and hang out in the bathroom, chatting. This morning was no exception.

When I came out of the shower, I discovered she'd drawn in the condensation on the window. Not a smiley face. Not her name. Nope, with this kid it's math she doodles, as often as not.

Lower line: -2 + 6 = 4
Middle line: -4 + 10 = 6
Upper line: 1 + 6 = 7