Try this:
Cpo = -0.75;a = 3.32;
M = 0.2:0.01:0.8;
Cp = Cpo./sqrt(1-M.^2);
Cp_cr = 2./(a*M.^2) .* ((((2+(a-1)*M.^2)/(a+1)) .^(a/(a-1)))-1);
xisx = interp1((Cp - Cp_cr), M, 0);
yisx = Cpo./sqrt(1-xisx.^2);
figure
plot(M, Cp)
hold on
plot(M, Cp_cr)
plot(xisx, yisx, 'pg', 'MarkerFaceColor','g')
hold off
set(gca, 'YDir','reverse')
legend('Cp', 'Cp\_cr', 'Intersection')
It would have been easier wit your actual code, rather than aan image of it.
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