error C2064: term does not evaluate to a function taking 2 arguments

Hey guys, I'm trying to create a little program here, but I'm having some trouble with it.

I'm getting the following error message: error C2064: term does not evaluate to a function taking 2 arguments. I don't really kn ow what the hell that means, would someone mind pointing out what the problem is so I can fix it? I was under the impression that a function as an argument was perfectly cool to do. Anyway, here are the two offensive lines, followed by the function definitions.

Code:

` `

//bad lines

x += delta_x((velocity_x(user_velocity, DEGREES)), t);

y -= delta_y((velocity_y(user_velocity, DEGREES)), t);

//function definitions:

double velocity_x(int vel, int degrees){

return vel * cos(degrees * 3.141592654/180.0);

}

double velocity_y(int vel, int degrees){

return vel * sin(degrees * 3.141592654/180.0);

}

double delta_x(double vel_x, double time){

return vel_x * time;

}

double delta_y(double vel_y, double time){

return vel_y * time - (.5 * (9.8 * (time*time)));

}

Any help would be greatly appreciated, because I have no idea what is wrong.