Take the 10 line to Zhichun Road, to Metro, to Xizhimen, to Line 2, to Chegongzhuang, get off the subway and go west. The first crossing is south, less than one stop, Lucy.
But I suggest you take a bus, which is more convenient.
Route 1: From Renmin University, take bus No.814 (southeast gate of Tiantan Building Materials City) and arrive at Fuchengmen. About 8.33 kilometers.
Line 2: Take Bus No.4 (National Defense University-Qianmen) from Renmin University to Fuchengmen North, about 8.07 kilometers.
Route 3: From Renmin University, take Yuntong 106 (North Gate of Central Party School-Tiancun North Road) to Chegongzhuang South, about 7.63 kilometers.
Line 4: Take bus No.320 from Renmin University (west of Baofusi Bridge-Beijing West Railway Station), then change at ganjiakou Building and get off at 102 bus (bus hub of Zoo-Beijing South Railway Station) and arrive at Fuchengmen. It is about 8.34 kilometers.
Line 5: Starting from Renmin University, take the Olympic Line 7 (Beichen East Road Bus Station-Xizhimen), then change to Metro Line 2 (Xizhimen-Xizhimen) at Xizhimen and arrive at Chegongzhuang. About 7.47 kilometers.
Route 6: Take bus No.732 from Renmin University (Summer Palace-Luhaiyuan Community), transfer to bus No.685 at the east exit of Erligou (the bus hub of the zoo-the outer city), and arrive at Chegongzhuang West, about 7.24 kilometers.
Route 7: Take bus No.332 (Summer Palace-Zoo Bus Hub) from Renmin University, transfer to bus No.0/9 (Zoo Bus Hub-Cuilin Community) at Zoo Bus Hub, and arrive at Maweigou. It is about 7.66 kilometers.
Route 8: Take bus No.320 (Xiyuan-Beijing West Railway Station) from Renmin University, change to bus No.0/02 (tram) at ganjiakou Building (Zoo Bus Hub-Beijing South Railway Station) and arrive at Fuchengmen. About 8.22 kilometers.
Route 9: Take bus No.697 (Yihe Mountain Villa-East of Caihuying Bridge) from Renmin University, transfer to bus No.685 (Zoo Bus Hub-Outer City) at Beijing Exhibition Hall, and arrive at Chegongzhuang West. About 7.0 kilometers.
Route 10: Take bus No.717 from Renmin University (northeast of Wangzhong Road-east of Caihuying Bridge), and change at ganjiakou Building to bus No.0/03 (zoo bus hub-west of Beijing Railway Station) to Fuchengmen. About 8.38 kilometers