Physics

The key is to sketch the field contributions from each charge separately, then add them as vectors. The $+Q$ charge at $C$ pushes a test charge leftward (toward negative $x$), while the stronger $+2Q$ charge at $D$ pushes it downward (toward negative $y$). Since $+2Q$ is twice as strong and only slightly farther away, its downward push dominates the leftward push, so the net field points into the third quadrant.

The key is to sketch the field contributions from each charge separately, then add them as vectors. The $+Q$ charge at $C$ pushes a test charge leftward (toward negative $x$), while the stronger $+2Q$ charge at $D$ pushes it downward (toward negative $y$). Since $+2Q$ is twice as strong and only slightly farther away, its downward push dominates the leftward push, so the net field points into the third quadrant.