Depends on if you mean 3×√320 or ∛320.

3√320 - 3√135 = 3(√320 - √135)

      = 3(√(2⁵·5) - √(3³·5))

      = 3√5(4√2 - 3√3)

3 sq rt 320- 3 sq rt 135

3 sq rt 64* sq rt 5- 3 sq rt 9 * sq rt 15

24 sq rt 5- 9 sq rt 15