3.83: Descending expansion of $\left(2x - \dfrac{1}{4x^{2}}\right)^{n}$

The first three terms, in descending powers of $x$, in the expansion of $\left(2x - \dfrac{1}{4x^{2}}\right)^{n}$ can be written in the form $256x^{c} + ax^{5} + bx^{2}$, where $n$, $a$, $b$ and $c$ are integers. Find the values of $n$, $a$, $b$ and $c$.