19
In the first image, we are given 4 match sticks.
In the second image, we are given 7 match sticks. The difference between the match sticks is 3.
In the third image, we are given 10 match sticks. The difference between the match sticks again is 3.
If in the third image we are given 10 match sticks, and for every new image, there is another 3 match stick added, then in the sixth image, there must be 19 match sticks. Image 4: 10 + 3 = 13. Image 5: 13 + 3 = 16. Image 6: 16 + 3 = 19.
Answer = 19. Hope this helps. :)