Sinulla (note that the last letter is an a, not an ä*) refers to a singular you, and teillä to a plural you. So if you got the English sentence, both 'sinulla'and 'teillä' should be accepted, but if you got the Finnish sentence, there is a difference in meaning, though the translation is the same.
*Though 'sinä' has an ä in it, the stem that is used to attach the adessive-ending -lla/-llä to, is 'sinu-'. Because of vowel harmony and the u that is in the stem, the ending becomes -lla. If the word/stem has 'a', 'o' or 'u', the ending will also use those same 'back vowels' (a/o/u), and if the word/stem has 'ä', 'ö', 'y' or ONLY 'i' and/or 'e' (so no a, o, u, ä, ö or y), the ending uses 'front vowels' (ä/ö/y).