You simply have to sum the two fraction together to get the total amount of occupied plot. To do so, multiply both numerator and denominator of each fration by the same number, so that they end up having the same denominator.
You can multiply both numerator and denominator of 1/8 by 3 to get 3/24. Similarly, you can multiply both numerator and denominator of 1/6 by 4 to get 4/24.
Now we can easily sum the two fractions together:
So, if 7 parts out of 24 are occupied, it means that there are parts left for other crops. So, the answer is 17/24.