Jess - you have correctly initialized theta according to the problem but you do not have to use it on the left side of the assignment (and leads to the above error when you try to since theta contains many values that are not integers). If you want Function_x to be a vector of elements that depend upon theta then just initialize this variable as
Function_x = 981./cosd(theta);
The same would be true for Function_y and Function_total. (I'm a little unclear on the naming of all three variables so you may want to rename them to something that more appropriate to their purpose.)
Try the above and see what happens!
