The three side lengths are x, x-2, x+8. The perimeter is these added together: 3x+6.
If the perimeter is ≥30cm then 3x+6≥30. Divide through by 3: x+2≥10, subtract 2 from each side: x≥8cm. So the base must be at least 8cm long. Minimum side lengths: 8cm, 6cm, 16cm.