！
Think it over calmly!
Literal
Calm-ly think [try-てみろ] [informative-よ].
冷静に ('calmly') is the adverbial form of 冷静 ('calm, composed'). ~てみろ is a rough/casual imperative form of ~てみる ('try doing') — the imperative みろ (from みる) gives it a brusque, commanding edge. よ at the end marks assertion/emphasis. A forceful urging, often addressed to someone being emotional.