II.2.2

This problem has Hartshorne Height 2. II.2.1 can be used to solve this.

HAPPY

 * For $$x \in U \subset X$$ know there is an affine containing $$x$$. Use that distinguished affines form a base for the topology of affines to show something like  $$ x \in D_f \subset \mbox{Spec} A \subset X$$ and $$ D_f \subset U$$.
 * The result follows from II.2.1.