Expanding the multinomial, the general term is 8!/i!j!k! * x^(3j+4k) for all i + j + k = 8.
The number of terms would be the number of distinct powers of x, the number of distinct outputs of 3j+4k with the specified constraints for i, j and k.
I attempted to make cases. 3j+4k where j+k <= 8...