Domain values represent possible x values that are allowed to be plugged in and produce a y value. The values (in this case value) that are not allowed to be plugged in are what make the denominator zero (since you cannot divide by zero). Simply set the denominator equal to zero to figure out this value.
3x + 8 = 0
3x = -8
x= -8/3, this value is not in the domain.
