Let's recall that rational numbers are those numbers that can be expressed or made by dividing two integers.
Example:
1.75 is a rational number because we can express it as a fraction : 7/4
0.33333 is a rational number because we can made it as a fraction: 3/9
√3 is not a rational number because we can't express it as a fraction
Upon saying that, we have:
6 + 864
870
870 can be made as a fraction: 870/1, therefore it is a rational number.