I suggest trying combinations of numbers and seeing what works. "Natural numbers" are positive integers, although some people define them to include the number 0 as well. So let's try:

Case 1: Let x+y = 7. Possible combinations for x & y are: 0 & 7, 1 & 6, 2 & 5, 3 & 4, 4 & 3, 5 & 2, 6 & 1, 7 & 0. Of these, which ones (if any) have the property that x^2 + y^2 <=30?

Case 2: now try x + y = 8. Posible combinations for x & y are: 0 & 8, 1 & 7, 2 & 6, 3 & 5, 4 & 4, etc. Again, which if any of these have the property that x^2 + y^2 <=30?

You should be able to see that the sum of the squares is smallest if x=y, which means for x+y = N that x and y both equal N/2. So once N gets large enough that (N/2)^2 > 30 there's no point in checking larger numbers.