Carlos Vela had an assist and provided a spark after starting the game on the bench for the home side, which is 19-7-3 with 60 points this season. Arango has scored five goals in three career matches against RSL.
LAFC beat Real Salt Lake for the sixth straight time and improved to 10-2-0 all-time in the series, including playoff matches.
Salt Lake (11-9-9, 42 points), which is tied with the Portland Timbers for the sixth-most points in the Western Conference, had a four-match unbeaten streak halted. The stretch began after RSL lost 4-1 to visiting LAFC on Aug. 6.
Zac MacMath made four saves for Salt Lake, which was outshot 20-8. LAFC put seven shots on target to RSL's five.
LAFC scored in the 49th minute after MacMath came out a few feet to break up a cross. The loose ball went toward Hollingshead, who sent a left-footed blast into the net for his fifth goal of the season.
Vela entered the match in the 58th. Just three minutes later, he sent a rocket left-footer just over the top of the net.
Arango made it 2-0 in the 68th minute with Vela delivering a pass. Arango got MacMath to the ground and easily sent a right-footer into the net for his 15th goal of the season.
Salt Lake nearly cut its deficit in half in the 73rd minute, but Crepeau stopped Anderson Julio's close-range header.
Vela took a penalty kick in the first minute of second-half stoppage time, but MacMath guessed correctly by moving to his left to stop Vela's left-footed blast.
LAFC had an 8-1 edge in shots in the first half.
Jesus Murillo had a solid chance in the 28th minute for LAFC, but his header was stopped by MacMath.