Hi Malvi
calculate no of moles for each element and convert it to simplest ratio.I am sure till this point you must be correct .
Al =1
S = 1.5
O = 6
Now here S is 1.5 which is can neither be approximated as 1 nor as 2 so what we will do -multiply all numbers by 2 to get whole number.
Al =1 X2 = 2
S = 1.5 X2= 3
O = 6 X2= 12
In the quiz answer should be Al2S3O12
I hope you agree with the above formula .
I have edited the formula in the quiz also.
For the second wrong question which you attempted in AP quiz in  chemtopper courses
Convert mass of P4----moles of P4------moles of H3PO4 ------mass of H3PO4
whatever mass will come that is the theoretical yield
take ratio of actual with theoretical and multiply it by 100%
I am getting theoretical yield somewhere near 65 g
Just try once more .

2nd one

What is the name for Fe2(CO3)3
I got it wrong on the quiz

And also, what is potassium trinitride? How is that K3N? Wouldn't it be KN3?

Fe2(CO3)3----Iron(III)carbonate

Actually it should be Potassiumnitride
K3N