Defining a new (informal) operator in CTL
You seem to be stuck, so here is the answer:

You want $a$ to hold for at least as long as $b$ does. The alphabet consists of $2^{\{a,b\}}=\{\emptyset, \{a\},\{b\},\{a,b\}\}$.

Rephrasing the condition, it means that $\{b\}$ cannot appear before either $\{a\}$ or $\emptyset$ have appeared. Indeed, this is the only violation of the condition.

So once $\{a\}$ or $\emptyset$ hold, you're in the clear. You just need to make sure that until then (sounds familiar?), $\{a,b\}$ holds.

Thus, you have
$$a\ Wh\ b\equiv (a\wedge b)U(\neg b)$$