If there are no values, then there is no plot, since plots require numbers.
If you have values, make the appropriate substitutions here:
P = 42; % Create Variable Value
a = 5; % Create Variable Value
M = @(x,P,a) P*a/pi*integral(@(alfa) alfa.*cos(alfa.*x./a) ./ (alfa.^3+1), 0, Inf, 'ArrayValued',1);
x = linspace(0, 100);
figure
plot(x/a, M(x,P,a)./(P*a))
grid
to get the result you want.
