View Compilation Error

0_0_11793433_6184.cpp: In function `double angle(Point, Point)':
0_0_11793433_6184.cpp:47: error: `length' undeclared (first use this function)
0_0_11793433_6184.cpp:47: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_11793433_6184.cpp: In function `double length(Point)':
0_0_11793433_6184.cpp:50: error: `double length(Point)' used prior to declaration