i am trying to save costs on a new 75 gallon aquarium and have a garage 48" lighting fixture with a 120 watt ballast and i two slots for bulbs, my question is what bulbs are best for a low to medium light planted aquarium (specifically i will be growing dwarf hair grass). what is the best combination of bulbs?

nearly every light is fine, plants dont care about spectrum, as long as its not blue. and 120 watts over a 320 litre tank should be ok to grow Eleocharis parvula, but i can tell you from personal experience with this plant, it is hungry for CO2 like all other carpeting plants so make shure your co2 is up to scratch.
fluorescent lights…..it’s the one that when you turn it off it turns blue