View Compilation Error

0_0_4716810_28486.cpp: In function `int intersect(Point, Point, Circle, Point&, Point&)':
0_0_4716810_28486.cpp:87: error: `my_sqrt' undeclared (first use this function)
0_0_4716810_28486.cpp:87: error: (Each undeclared identifier is reported only once for each function it appears in.)