(1/2a^4 x²-1/3a³x²-3/4a³x^4)÷(-2/3a²)
＝ (1/2a^4 x²)÷(-2/3a²) -(1/3a³x²)÷(-2/3a²) - (3/4a³x^4)÷(-2/3a²)
= -3a²x² +(1/2)ax² +(9/8)ax^4