This expression is a "formula" because it has a well-defined mathematical value (for each allowable set of values for the variables in it). It is a "statement" because that mathematical value is either "true" or "false". But an "equation" is a statement claiming that two expressions are equal to each other (equal $\equiv$ equation), and there is no "=" sign in this statement. Therefore it is not an "equation".
Examine when your statement will be true. It has three parts joined together by ands:
$$(\lnot a\lor b)\\(a\lor b)\\\lnot a$$
For the whole expression to be true, all three must be true. But the last one requires $a$ to be false. And if $a$ is false, the first will be true, regardless of $b$. Finally $a\lor b$ requires either $a$ or $b$ to be true. But we already know $a$ has to be false, so $b$ has to be true.
Therefore, this will be true only if $a$ is false and $b$ is true.