[ADV 副词]（用于形容词和副词前）非常，极其 You use extremely in front of adjectives and adverbs to emphasize that the specified quality is present to a very great degree.
[ADV adj/adv]
[emphasis]
My mobile phone is extremely useful...
我的手机非常有用。
These headaches are extremely common...
这类头痛非常常见。
Three of them are working extremely well.
他们中有3个人工作非常好。
Oxford
ex·treme·ly★/ɪkˈstriːmli; NAmEɪkˈstriːmli/adverb( usually with adjectives and adverbs 通常与形容词和副词连用) to a very high degree 极其；极端；非常◆extremely important/useful/complicated极为重要╱有用╱复杂◆She found it extremely difficult to get a job.她发觉找工作极其困难。ex·treme·ly/ɪkˈstriːmli; NAmEɪkˈstriːmli/