in a survey of 36 students it was found 30 play tennis, x play volleyball only 9x play both tennis and volleyball 4 play neither tennis nor

I would do it CPhill's way but I would use a venn disagram to help me.

I have just started it here.   

4 play neither

x play volleball only

9x play both tennis and volleyball

And (30 -9x) play tennis only

So we have

4 + x + 9x + (30 - 9x) = 36

34 + x = 36

x = 2 = those who play volleyball only

So 9(2) = 18 play tennis and volleyball

And (30 - 9(2))  = (30 -18) = 12 play tennis only

And

4 + 2 + 18 + 12 = 36