a)
承担, undertake( some duty), do (some work).
担任, work as (the chairman/ the team leader).
Here we have 主持人the host, so we'd better say either
承担了广播站的主持人工作
or担任了广播站的主持人。
b)
昨天雨下得很大,赶走了连续几天来的高温。
Yesterday the rain poured heavily and drove away the several-day hear