c is double, thus you cannot use the modulo operator %.
Use fmod() instead.
So change this:
c %= 3
to this:
c = fmod(c, 3);
As Slava mentioned, you could have used an int instead, like this:
int c = 5; // for example c %= 3
which wouldn’t require the use of fmod(). It’s important to understand that the modulo operator % works with ints.
As πάντα ρέι mentioned, there is also this: Can’t use modulus on doubles?
As a side note Victor, you have so many variables, but most of them are unused and or uninitialized. Did you compile with all the warnings enabled? Here is what I get when compiling your original code (with the line that generates the error commented out):
C02QT2UBFVH6-lm:~ gsamaras$ g++ -Wall main.cpp
main.cpp:9:5: warning: expression result unused [-Wunused-value]
c, b, x, y, z, a, c = 100;
^
main.cpp:9:8: warning: expression result unused [-Wunused-value]
c, b, x, y, z, a, c = 100;
^
main.cpp:9:11: warning: expression result unused [-Wunused-value]
c, b, x, y, z, a, c = 100;
^
main.cpp:9:14: warning: expression result unused [-Wunused-value]
c, b, x, y, z, a, c = 100;
^
main.cpp:9:17: warning: expression result unused [-Wunused-value]
c, b, x, y, z, a, c = 100;
^
main.cpp:9:20: warning: expression result unused [-Wunused-value]
c, b, x, y, z, a, c = 100;
^
main.cpp:10:5: warning: variable 'x' is uninitialized when used here [-Wuninitialized]
x += 5;
^
main.cpp:7:16: note: initialize the variable 'x' to silence this warning
double b, x, y, z, a, c;
^
= 0.0
main.cpp:11:5: warning: variable 'y' is uninitialized when used here [-Wuninitialized]
y -= 2;
^
main.cpp:7:19: note: initialize the variable 'y' to silence this warning
double b, x, y, z, a, c;
^
= 0.0
main.cpp:12:5: warning: variable 'z' is uninitialized when used here [-Wuninitialized]
z *= 10;