From: United States
You need to focus on the puzzle as a whole. Often times a particular location in the puzzle is not solvable until you've first filled in other areas. The location where your red box is cannot yet be determined. And I'm not sure how you concluded which boxes surrounding the 3 & 4 at the bottom edge should be filled. Your first move in this puzzle is to fill-in 2 squares adjacent to the 5 on the upper edge and on the left-side edge. We know these squares are filled because of the remaining 4 squares only 3 can be filled. Those 3 plus these 2 gives us the 5 we need. Your next move will be to consider the areas surrounding the 3s on the upper and left side edge. You know that the 3 filled-in squares MUST land within the group of 4 squares shared by the 5, therefore you can confidently cross out the other 2 squares.