View Compilation Error

0_0_8830601_7403.cpp: In function `double angle(const Point&, const Point&, const Point&)':
0_0_8830601_7403.cpp:140: error: `length' undeclared (first use this function)
0_0_8830601_7403.cpp:140: error: (Each undeclared identifier is reported only once for each function it appears in.)