 <p><strong>1.</strong> Your function signature takes 3 arguments, and you only supply 1. How it compiles, I do not know, as I don't think it should. </p> <p><strong>2.</strong> The <code>CalculateAirTime</code> function prints a variable before calculating it. You first do this: <code>printf("Total air time: %f\n", time);</code> , and then calculate the actual air time: <code>time = ((2*velocity)*sin(angle)/GRAVITY);</code></p> <p><strong>3.</strong> As an advice, since the <code>CalculateAirTime</code> function needs velocity and angle to return a calculated time, do not pass time as a parameter. It's useless. You can either declare a double variable inside the function or directly return the calculated time.</p> <p>Replace <code>double CalculateAirTime(double velocity, double angle, double time){...}</code> with <code>double CalculateAirTime(double velocity, double angle){...}</code></p> <p><strong>*EDIT!</strong> For clarity purposes, here is how I would probably write your program:**</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;math.h&gt; #define PI 3.14159265358979323846 #define GRAVITY 9.8 /* I renamed some variables. Just from the variable names, I think you do not understand the solution for this "air time" problem well. I might be mistaken tough. If you have any questions, please ask them. You are not leaving this place until you fully understand what is happening. */ // Function DECLARATION: double CalculateAirTime(double Initial_Velocity, double Launch_Angle); int main(void) { // variables double Initial_Velocity; double Launch_Angle; // Initial_Velocity = 20.0 should work aswell. This is called a cast. You will learn about it later Initial_Velocity = (double)(20.0); // Converting 45 degrees to radians, because the sin() library function works with radians Launch_Angle = (double)((45.0/180.0)*PI); //Call function using our declared values as input CalculateAirTime(Initial_Velocity,Launch_Angle); return 0; } // Function DEFINITION //Try to use the same name for paramaters here, as you did in the declaration! double CalculateAirTime(double Initial_Velocity, double Launch_Angle) { double Air_Time; Air_Time = (double)((2*Initial_Velocity)*sin(Launch_Angle)/GRAVITY); printf("Total air time: %f\n", Air_Time)); return time; } </code></pre>