I'm not sure why my previous answer was deleted, but heres another attempt.
The vector which opposes the vector with components [-4,3] would be [4,-3], because taking the opposite of each component gives the opposite direction. the length of [4,-3] (your new vector) is 5 because √((4)²+(-3)²) => √(16+9) => √25 = 5. This is half the necessary length, so to fix this we need to double both components, giving [2*4,2*(-3)] = [8,-6]
Final answer:
The vector with components [8,-6] has a length of 10 and opposes the direction of [-4,3].
I hope this helped clear up any confusion over my last answer. :)
