calculate CFM of AHU

You are going to have to provide a lot more information if anyone is to help you.

For starters, what is an AHU?

AHU means Air Handling Unit. This is not a refrigeration unit, whose output is rated in tons. AHU's are made in assorted sizes and their capacity is rated in CFM meaning Cubic Feet per Minute of air. So your AHU has a rating, just as a motor has a rating and a lightbulb has a rating. And it should have a nameplate. And the CFM rating should be on the nameplate. So why not read the nameplate or call the manufacturer and ask what CFM this AHU is rated for. All of this makes the question rather peculiar.

The OP needs to clarify. And is the replying guest the same person? If the AHU is rated in tons, it almost surely incorporates refrigeration, and possibly reheat, make-up air, and bypass air among its functions.

In round numbers, cfm = tons × 10,500 ÷ Δt of air.

how to find Δt of air????

Measure the inlet and outlet air temperatures with a thermometer; subtract.