View Compilation Error

0_0_19501193_28248.c: In function 'main':
0_0_19501193_28248.c:41:1: error: expected expression before '/' token
 //        printf("x1,y1 %d,%d x2,y2 %d,%d\nsum %.3lf\n",x1,y1,x2,y2,sum);
 ^
0_0_19501193_28248.c:46:1: error: expected expression before '/' token
 //   printf("a %d sum %.3lf\n",a,sum);
 ^
0_0_19501193_28248.c:47:4: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
    for(int i=y1;i<y2;i++)
    ^
0_0_19501193_28248.c:47:4: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code
0_0_19501193_28248.c:49:1: error: expected expression before '/' token
 //   printf("3:%.3lf\n",a*X);
 ^