\text{Let $X$ be a Banach space and let $Y$ be a closed subspace of $X$.} Let $Y^\circ$ in $X^\star$ be defined by \[ Y^\circ = \{\ell \in X^\star : \ell|_{Y} = 0\}. \] Given a bounded linear functional $f$ on $X/Y$, define $\pi^\star(f) \in X^\star$ by $(\pi^\star(f))(x) = f([x])$. Prove that $\pi^\star$ is an isometric isomorphism of $(X/Y)^\star$ onto $Y^\circ$.