Simplifying with radicals: a(sqrt8) - 3(sqrt2a^2), etc

sometimes the easy ones throw me off... is this right?

1) a(sqrt8) - 3(sqrt2a^2)

2a(sqrt2) - 3a(sqrt2)

-a(sqrt2)

One other question if I may...

2) My teacher only gave me 75% on this question. Could anyone tell me why?

-cbrt(64)

My answer was -4.
Is that not right?

1) If you are supposed to assume that the variables are non-negative, so sqrt[a²] = a (rather than |a|), then yes, this appears to be the correct simplification.

2) Since we do not have the instructions, it is difficult to say. Were you supposed to find all three roots (the real one and the two complex ones)? Were you supposed to show the steps, or give the rules justifying the simplified form? Or something else?

In the end, of course, only your instructor can explain your instructor's grading process and philosophy.

Eliz.