Venn Diagram is not necessary but it is way much easier to figure the problem using it. This one consist of three circle.

-You know that 14 play both tennis and squash. Because 2 play all sports, 12 play ONLY tennis and squash.

-You know that 8 play both golf and tennis. Because 2 play all sports, 6 play ONLY golf and tennis.

-You know that 24 play tennis and that 12+2+6 play tennis and something else so 4 play only tennis.

Now you get this system of equations:

Let x be the number of people that play only golf, y the number of people that play squash and golf but not all three sports and z the number of people that play only squash.

18 = 6 + 2 + x + y

23 = 12 + 2 + y + z

39 = 24 + x + y + z

You have three equations and three unknown. You can solve for y.

Ans : y=4