Identification of known terms :

m = 3, f= 15 cm

m = -Di/ Do

Do = -Di/ 3 and di = -do/ 3

Now, we know that, 1/f = 1/do + 1/di

1/di = 1/15 – 1/do

-3/do = do – 15/ 15do

-3 = do- 15/ 15

do – 15 = -45

do = -30.

Position of object is 30 cm away from lens on left side.