Finding intersection of polyfit and line

I am trying to find the intersection between the polyfit of a line and the line y=1. I have the following code:

polyfit(l,FS,3)
int=roots(polyfit(l)-1)

Essentially, I want the value of length when the Factor of safety is 1:

Answer 1

[dpb]

Doesn't do any good to fit the data but not save the coefficients of the resulting fit...

b=polyfit(l,FS,3);     % Get coefficients of polynomial
[~,ix]=min(abs(FS-1)); % Nearest observed point to 1 is
L0=l(ix);              % good guess for starting point
LOne=fsolve(@(b,x) polyval(b,x)-1,L0);  % Solve for x where == 1

Answer 2

[Walter Roberson]

b = polyfit(l,FS,3);     % Get coefficients of polynomial
LOne = roots(b - [0 0 0 1]);
LOneReal = LOne(imag(LOne==0));
LOneInRange = LOneReal >= min(l) && LOneReal <= max(l);

This would be empty if there are no real roots in the range.

[Jan Ulbrich]

I think there is a typo and it should be "LOneReal = LOne(imag(LOne)==0);"