difficulty为不可数名词，所以前面的不定冠词a要去掉，另外前面已经有动词had，所以have也不能要。
great difficulty catching up with the classmates
应该是have difficulty in doing something吧
87. Soon after he transferred to the new school, Ali found that he had_____(很难跟上班里的同学)in math and English.
a great difficulty catching up with the classmates
你翻译对了！
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