You use extremely in front of adjectives and adverbs to emphasize that the specified quality is present to a very great degree.
exceedingly , very , highly , greatly
My mobile phone is extremely useful.
我的手机非常有用。
Three of them are working extremely well.
他们中有3个人工作极其出色。
These headaches are extremely common.
这类头痛极其普遍。