C :: How To Calculate Surface Area And Volume For Rectangular Prism

So for my assignment, I have to write code to calculate the surface area and volume for a rectangular prism that involves the use of functions. I made the program without functions and it works perfectly but as I'm putting in the required functions, it conversely made it non-functional, ironically. How to call functions correctly and the online book we're using now is confusing me even more.

Code:

#include <stdio.h>
#include <math.h>
int main(void) {
double x1, x2, x3, x4, x5, x6, x7, x8 = 0;
double y1, y2, y3, y4, y5, y6, y7, y8 = 0;
double z1, z2, z3, z4, z5, z6, z7, z8 = 0;
double length, width, height=0;

[Code]....

And here is the version I'm using to test it (without the need to input the individual coordinates).

Code:

#include <stdio.h>
#include <math.h>
int main(void) {
double x1, x2, x3, x4, x5, x6, x7, x8 = 0;
double y1, y2, y3, y4, y5, y6, y7, y8 = 0;
double z1, z2, z3, z4, z5, z6, z7, z8 = 0;
double volumePrism, surfaceArea =0;

[Code]...