The first two terms have 5x2 in common and the last two terms have 2 in common, so:
10x3 + 15x2 + 4x + 6 → 5x2(2x + 3) + 2(2x + 3)
Now 2x + 3 appears twice so we can go further and write
10x3 + 15x2 + 4x + 6 → 5x2(2x + 3) + 2(2x + 3) → (2x + 3)(5x2 + 2)
.