From Hong Kong, what is my best solution to get to Guangzhou airport? I have to be there by 5:20pm.

I am sorry that there is no subway service between them. You may take a train to Guangzhou East Railway Station from Hung Hom Railway Station (about 2 hours). Upon arrival, take subway line 3 to the airport.

Or, you can take a bus to China Hotel by Marriott from Marco Polo Hotel, Metropark Hotel Causeway Bay, Kwun Tong APM Station, etc. Upon arrival, take metro line 2 from Yuexiu Park to Jiahewanggang Station, and switch to metro line 3 to the airport. :)