How can I create a legend on the interception of 2 lines on a plot with its coordinates?

I have this code so far, I want to get the interceptions of plot(CrankAngle1,n1) and plot(xl(:)*[1 1], [1;1]*[0.5 0.9], '--r').

Any suggestions?

figure(1)
plot(CrankAngle1,n1)
hold on
xl = xlim;
plot(xl(:)*[1 1], [1;1]*[0.5 0.9], '--r')
hold off
title('Wiebe heat release fraction curve for n=1')
xlabel('Angle')
ylabel('Xb')
grid