How are you calling the function? You can't just press the green Run triangle in the editor for a function that requires input arguments. Go to the command window and call the function with an appropriate value for the porosity input argument.
It seems to work fine here (at least it runs and returns a result).
result = problemset4pt1(0.1)
function f_x = problemset4pt1(porosity)
% Constants
deltaP = 810.5; %dyn/cm^2
l = 20; %cm
mu = 0.01; %Poise
rho = 1; %gm/cm^3
dp = 0.1; %cm
u = 0.75; %cm/s
%Evaluating function
f_x = 150*mu*u/(dp^2)*((1-porosity)^2)/(porosity^3) + 1.75*rho*(u^2)/dp*(1-porosity)/(porosity^3) - deltaP/l;
end
