It means that you have to add 7 balls for 9 times. To express this situation, it will be 7ballsx9=63 balls
Frankly this is my interpretation. By the way, I did not expect the maths question in primary school to be interpreted like that. Personally I think either 7x9 or 9x7 will do the job since the question was asking how many balls and the answer is 63 from either ways. Perhaps I have to change my attitude of primary education and treat it very very seriously.