Answer :

so 3+x=4
3+1=4
4+x=6
4+2=6
notice the patern
3+1=4
4+2=6
6+3=9
9+4=13
13+5=18
18+6=24
so then next + must be 7
24+7=31

3  4   6  9  13  18  24

If you watch carefully, between the first two numbers  1 was added.

Then 2 was added to 4 to have 6.

3 added to 6 to give  9.

4 added to 9 to result to 13.

5 added to 13 to give 18.

6 added to 18 to give 24.

Therefore 7 would be added to 24 to give:  24 + 7 = 31.

So it is 31. Cheers.