I hope someone can help me! I was completely stuck, so I took a hint. The square that was filled in is the one with the arrow pointing to it, in the top middle. But I can't for the life of me figure out the logic that would have allowed me to get that! Can anyone explain it to me?
It's because the square next to it has to be part of either the 17 or the 7 horizonatally, and that square is necessary for both of those possibilities. If the square next to it is part of the 17, then that square obviously has to be filled to connect it. If the square next to it is part of the seven, it also needs to extend into the hint square, because if it only extended to the left, it would connect to the far left filled square and form 8.