Given two points...
#include <math.h>
CGPoint p0;
CGPoint p1;
The angle is the arctan of the quotient of the change in each dimension...
double dx = p0.x-p1.x;
double dy = p0.y-p1.y;
double angle = (dx)? atan(dy/dx) : NAN;
FYI, for next time, the SO culture strongly encourages questioners to show research, produce an attempt at a solution.