Those ought to throw an error. You can't multiply a function handle and a number. You can multiple the result you receive from evaluating a function handle and a number, however.
% Arbitrary values for the variables used in the anonymous functions
F1 = 2;
I1 = 3;
l1 = 4;
b = 5;
c = 6;
M1z = @(x) F1*x - I1*(x-l1/2);
% I assume bx should be b*x otherwise Ixx doesn't depend on x
Ixx = @(x) (b*x+c)^4/64;
% Evaluate M1z and Ixx at x and use the results in evaluating Sxx at x
Sxx =@(x) M1z(x)*(b*x+c)/2/Ixx(x);
fplot(Sxx,[0 1.5],'b')
This issues a warning. You probably want to vectorize Sxx and Ixx using element-wise multiplication, division, and power (the .*, ./, and .^ operators instead of the *, /, and ^ operators.)
