Birthday Cake

You baked an awesome, perfectly rectangular cake for your friend's birthday party. But before you serve it, someone cuts a smaller, rectangular piece out of the cake. With a single, straight cut, can you always divide the remaining cake into two equal halves (equal areas)? Devise a strategy to draw the line and cut along it.

Note: your adversary may have cut out the piece in any way - could be a random orientation and/or along the edge, but the cut will be still be a rectangle. In the image below, the colored rectangles are some different examples of what the inner cutout could look like. Your strategy for cutting a single straight line should work for all of these cases.

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