Check if you have shadowed any function with a variable. Simply by running your code in a clean workspace I found some typos. The code below runs without issue.
In debugging situations like this, check if using clear at the start of your code helps. If it does your errors are due to old variables in your workspace.
% Values of constants
l = 89;
h = 49;
D = 55;
beta1 = 11.5;
alpha = 33;
% Compute A, B, C, E
A = l*sind(beta1);
B = l*cosd(beta1);
C = (h+0.5*D)*sind(beta1) - 0.5*D*tand(beta1);
E = (h+0.5*D)*cosd(beta1) - 0.5*D;
sindalpha = sind(beta1);
cosdalpha = cosd(beta1);
% output
output = A*sindalpha*cosdalpha + B*sindalpha^2 - C*cosdalpha - E*sindalpha;
disp ('Part a)')
fprintf('%f\n',output);
