HELP! PLS! ASAP! Back titrations

[_cheers]

Ok, I love Chem, but I cannot get these back titrations! I have a lab question and I'm really stuck.

Its determining the molar mass of an unknown metal carbonate: 1.85 g

this is diluted with distilled water up to 250 ml, then 20 ml of solution is mixed with 15 ml of .1789M HCL. This is then back titrated with 17.56 ml .1013M NAOH.

I kow I need to find the acid added, the acid unused to find the acid used, but I keep getting the wrong numbers. I'm not sure what to make of the dilution into 250 ml or where to add it into the equation...

HELP! :shock: