Extrude the green edges downwards as show by the red arrows.
Next create a new polygon at the rear wheel arch. Now shape the rear wheel arch the same way that we did the front wheel arch at the beginning of the tutorial. Just do some extrusions and vertex adjustments as necessary in all viewports to get the correct shape as shown by the red outlined polygons and green arrows.
Now we have to make the rear wheel arch part of the same object as the rest of the car since they are two separate objects right now. Select the body of the car and select "Editable Poly" in the stack. Click the button labeled "Attach" and then select the rear wheel. This makes the rear wheel arch part of the rest of the car.
Now that the rear wheel arch is connected, we need to bridge the gaps with new polygons. We are going to use a new tool for this. Select "Polygon" from the stack and then lower down the panel, click the "Create" button. Now starting at the bottom, make the lowest red polygon by clicking the surrounding vertices in an anti-clockwise order as shown by the vertices i.e. click the bottom left vertex, followed by the one to the right of it, then the one above, then the one to the left of that and then back to the first one you clicked. Do the same for the other two polys shown in red.
Make the extrusions shown in the picture making sure you adjust vertices in all views to get a good shape.
Make the cuts shown in red. Delete the poly shown in green. Target weld vertex 1 to vertex 2.
Create the two red polys like we did previously. Make sure to create them by clicking the vertices in an anti-clockwise order otherwise the normals of the polys will be reversed.
Make the extrusions shown in the same was as we have been doing and adjust vertices as required.