Can't understand a given answer about a 1st Kirchhoff's law question
Maybe what you are missing is that current has a direction. When you write your equation you assume a given flow. You assume that I6 and I4 are flowing into the node (so you sum them), and that I5 is flowing out of the node (so you subtract it).

Depending on how they actually flow, one or more of your currents will come out as negative. This is not a problem, it's just relative to the direction.

The book does not pick the same assumption, and assumes that they all flow into the node. They will most certainly get at least one negative result.

No matter which one you pick, it's very important to track these signs.