Your case statements are not valid. Cases use constant values, not expressions. Your code should look like:
int x,y;
float z;
printf("Write 2 integers: ");
scanf("%d%d",&x,&y);
if (x+y>0 && x+y<=4)
{
z = exp(x+y);
}
else if (x+y==8 || x+y==10)
{
z = (pow(x,2)+y)/(x-y);
}
else
z = 0;
printf("%f is an output.\n",z);