I got as far as \begin{align} w \vDash \neg (a U b) &\Leftrightarrow \neg (w \vDash a U b) \Leftrightarrow \neg (\exists_{i\geq0} : w^i \vDash b \land \forall_{0\leq k < i} : w^k \vDash a) \\ &\Leftrightarrow \forall_{i\geq0} : \neg(w^i \vDash b) \lor \exists_{0\leq k < i} : \neg(w^k \vDash a) \end{align}
but got stuck.
If you could offer some advice as on where to start I would very much appreciate it. Thanks in advance