(b/2a)^2 /(-b/a) ×(-3b/4a)^3
= (b/2a)^2 × (-a/b) ×(-3b/4a)^3
= b^2 × a^-2 / 4 × 27a^-2/64 × b^2
= 27b^4 / (256a^4)
或
= [3b/(4a)]^4 / 3