Answer
$4x^4-6x^2-\dfrac{3}{4}x+8$
Work Step by Step
Since the expression in parenthesis is preceded by a negative sign, then all terms inside the parenthesis will alter its sign when the parenthesis is removed. Hence, the given expression, $
-\left(-4x^4+6x^2+\dfrac{3}{4}x-8\right)
$, is equivalent to
\begin{array}{l}
4x^4-6x^2-\dfrac{3}{4}x+8
.\end{array}