have a solution with 600mg/L of Al and 4200 mg/l of Na

How many grams of Al2(SO4)3 and Na2SO4 will produce this solution

600mg Al ions = 0.6g Al ions per litre. If Al= 27, then this is (0.6/27) moles Al ions. Based on the formula this must be equivalent to 0.5 x (0.6/27 moles Al2(SO4)3 as you need 2 moles Al ions per mole aluminium sulphate. So find the molar mass of aluminium sulphate and then find the mass of 0.5 x (0.6/27 moles Al2(SO4)3.

600mg Al ions = 0.6g Al ions per litre. If Al= 27, then this is (0.6/27) moles Al ions. Based on the formula this must be equivalent to 0.5 x (0.6/27 moles Al2(SO4)3 as you need 2 moles Al ions per mole aluminium sulphate. So find the molar mass of aluminium sulphate and then find the mass of 0.5 x (0.6/27 moles Al2(SO4)3.

Do a similar calculation for the other salt.