You may need to play with a "pressure" on the back cover while you trying to slide it. Usually you put both hands on the back cover, put your thumbs on the front of the laptop (little bit below the edge of the back cover), and try to push it with your thumbs.
And here is Lenovo manual, check page 34 and down:
https://download.lenovo.com/consumer/mobiles_pub/lenovo_g_z_50_series_hmm.pdf
Good luck,
Mike | Headset Adapter Co.
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