Maths inquiry-Mr john

Hey this is my maths inquiry based on mr john. This is what it says,It sounds like Mr John is not having a good day! He wonders if you can help him to solve this maths problem as his class are not so good at counting. Try look at this picture to understand a bit more. and the question is so **4 + 28* *= a missing and you can use digits from 0123456789 only once and I finally use trial and error to get the answer.