Formula for Finding A Portion of SG

This might have been discussed before, but after googling quite a bit, maybe I'm just asking the question incorrectly.

I am brewing a version of Brandon O's Graff. So I have an extract wort and I add some juice to the carboy in place of water. (To bring to 5 gallons)

I failed to take a reading of the original wort before adding the juice. I was wondering if someone knew of some sort of formula (I am horrible with math) to determine the original gravity of the wort before juice?

I don't have a final gravity as of yet since it's not done...but I was thinking there might be some way to just plug in a FG and roughly determine the wort OG. Hope that makes sense.

Something like this? (again, not good with math)

Final Gravity of Wort/Must Combined: 1.012 (could change)
Original Gravity or Apple Juice: 1.050 (constant)
Original Gravity of Wort/Must: 1.070 (includes original wort + apple Juice : constant)
Original Gravity of Wort Only: (??????)

There was 3 gallons of Wort and 2 gallons of apple juice added to the carboy to get the OG of 1.070.

I can provide more details if anyone is interested. Thank you SO MUCH in advance! Maybe this isn't even possible to determine.

Volume of Juice * gravity of juice + volume of wort * gravity of wort = total volume * total gravity

2*(1.050) + 3(???) = 5(1.070)

So, your wort gravity was 1.083.

wow. Thank you very much! I knew someone in this community would be able to do this easily. Last night, my brain caught fire while trying to figure this out and I had to extinguish it with homebrew. (that made matters worse)