How to solve graphed equation at specific point?

Here's my code and I want to find the value of wavelength where it crosses 0. I can see from the graph its around 1.3. I'm not sure what command to use to solve for this value?

>> A=1;

>> c=299792458;

>> gk1=0.69617;

>> gk2=0.40794;

>> gk3=0.89748;

>> lk1=0.0684;

>> lk2=0.1162;

>> lk3=9.8962;

L=sym('wavelength');

n1=(gk1*L^2)/(L^2-lk1^2);

n2=(gk2*L^2)/(L^2-lk2^2);

n3=(gk3*L^2)/(L^2-lk3^2);

n=(n1+n2+n3+1)^(1/2);

d2=diff(n, L, 2);

>> GVD=-L/c*d2;

>> ezplot(GVD*10^12,[0.5,2.5]);

>> grid on;

>> ylim([-500 100]);