We can use the method of trial and error by choosing appropriate totals for the whole magic square and filling the magic square in to see if it works.
When choosing the numbers to use, you have to think logically, the number must be above 24 and less than 42. Those are the smallest and largest numbers you can make with these numbers. 24 = 7+8+9 and 42 = 13+14+15
A number around 30 seems very promising so You should start there. The reason I advise this is that 42 can only be made one way and 24 can only be made one way. So the numbers close to them will also be too high or low. Work systematically and analyse why it didn't work if it didn't. Do you need to increase the number or decrease it. You should end up with the magic square with a total of 33 and N=8.