how to find the intersect between the curve and the straight line ?

close all;
clc;
clear all;
files={
'B1586-IIA 14.2 2.00 303 T10 26.01mA -8.6.txt'
};
for t=1:size(files,2)
    h=dlmread(files{t},'\t', 5, 0);
    x = h(:,1);
    y = h(:,2);

      k=y;
      l=x;
  end
  fig=figure();
  set(fig,'color','white');
  plot(x,y,'b*');
  xlabel('frequency (GHz)');
  ylabel('Response(dBm)');
  hold on 
  X=[];
  N=14;
  for i=0:N;          % Making the Y=X*phi making X Matrix 
      X=[X,l.^i];
  end
  phi=inv(X'*X)* X'*k;
  Y=X*phi;
  c1=plot(x,Y,'g-','linewidth',2);
  hold on
  initialY=-50.59;
  c2=plot([0 14e9], [initialY-3 initialY-3],'y-','linewidth',2);

C1 is the Curve and C2 is the -3db line

how to i find the intersection between the curve and the -3db line ?