The three rows of a grid follow one and the same rule. Row 1 reads 4, 7, 17. Row 2 reads 6, 5, 19. Row 3 reads 8, 3 and then a gap. What number belongs in the gap?

In each row, multiply the first two numbers and then take away their sum. Row 1: $4\times 7=28$ and 4 + 7 = 11, so 28 − 11 = 17. Row 2: $6\times 5=30$ and 6 + 5 = 11, so 30 − 11 = 19. Row 3: $8\times 3=24$ and 8 + 3 = 11, so the gap holds 24 − 11 = 13. The traps: 24 is the product with the subtraction forgotten, 35 adds the sum instead of subtracting it, and 11 is just the sum.