‎This is a quote from my English textbook.

"Once he decides he wants to do something a certain way,he won't listen to any advice or suggestions."

in this sentence,why isn't a preposition before the "certain way"

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"a certain way" is describing how "he wants to do something" so it acts like an adverb.
You can replace it with other adverbs, so it becomes clearer.
"Once he decides he wants to do something quickly, ... "
"Once he decides he wants to do something slowly, ... "

A preposition usually shows a direction, time, or location, which is not the meaning of "a certain way" here.