11) Place the 150 mm lens about 4.0 cm after the crossed-arrow object, and then place the 75 mm lens about 6.0 cm after the 150 mm lens. Finally, place the screen after the 75 mm lens and locate a focused image by moving the screen back and forth. Measure and record the distance between the crossed-arrow object and the 150 mm lens (the first lens), the distance between the lenses, and the distance between the 75 mm lens (the second lens) and the location of the image. Note that the first value is the object distance for the first lens, and the last value is the image distance for the second lens. Also, measure and record the height of the image on the screen. This value is the image height for the second lens. You may want to apply any sign conventions for these quantities, if necessary.

For the multi-lens system in step 11, use the thin lens equation for each lens, the data you gathered, and the expected focal length of the converging lens to calculate the focal length of the diverging lens.

The data that was gathered in step 11 is the following:

Expected focal length of diverging lens: -150mm

Distance between the crossed-arrow object and the 150mm lens (first lens): 40mm

Distance between the lenses: 60mm

Distance between the 75mm lens (second lens) and the location of the image: 420mm

Height of the image on the screen: 58mm

How do I use this information to calculate the focal length of the diverging lens?