2001 Malibu A/C high/low pressure lines?

I have a 2001 Malibu, 6 cyl and need to recharge the a/c unit. I see 2 lines, one with a larger black cap on it in open view in front of the engine and another line with a smaller blue cap way down in back of the engine, down low near the firewall. I need to know which one is the low pressure line. Please be sure of your answer. I don't need to have any accidents. Thanks.