Troubles understanding this Interval Scheduling question
You'll want to start by answering the question in the hint, and then make another argument to obtain the final result.

The hint is saying that for any job $j \in A$, there is a fixed upper bound on the number of intervals in $B$ with which $j$ overlaps. Think about what happens if $j$ intersects with three or more intervals in $B$. Would this be possible using the Shortest-Job-First algorithm?

Let the upper bound you obtain from the hint be $k$. Then, you can make the following argument (presented informally, as it is just a hint). Using our result above, we can state that in the worst case, we have each $j \in A$ overlapping with a subset of $k$ distinct jobs of $B$, where these subsets of $k$ jobs are pairwise disjoint. This gives you an upper bound on $|B|$ in terms of $|A|$.

Can you take it from here?