75' - hire a kid or rent a ditchwitch. Personally I would dig once, put in pvc conduit with the proper elbow radiuses and number of degrees of bend max, then use a vacuum to **** a string through it and pull the cable. Not only will it be better protected, but it will be easy to replace if need be. I have a 40' run of many conductor sprinkler cable I did that way 25 years ago. One segment died and there were no spare ones to use. It took us maybe 30 minutes to pull a new cable a month ago. I was so thankful that I put that conduit in, some of it is under a concrete walk now. But if you are looking for cheap and easy only then use a shovel to dig a 3 or 4" deep Vee in the soil, put the cable in the bottom of the Vee and cover it. Don't waste your time digging 6" deep in a trench if you don't mind replacing it again when someone hits it accidentally putting in a shrub, or some ground squirrel / mole decides to take a chomp.