If you ask for two outputs from the max() function you can find the index of your peak:
P=zeros(1,750);
theta=zeros(1,750);
L=zeros(1,750);
F=zeros(1,750);
W=10;
g=9806;
for x = 1:750
theta(x)=atand(400/x);
L(x)=sqrt((x^2)+(400^2));
F(x)=W*g*cosd(theta(x))*500;
P(x)=F(x)/L(x);
end
plot(1:750,P)
xlabel('x in mm')
ylabel('P in kg/mm/s^2')
[Pmax, index] = max(P);
fprintf('The maximum P value in kg/mm/s^2 is: %0.2f at x = %d mm\n', Pmax, index);
