40058

3 ways of forming 707 by adding a number to its reversal:

By using the rules:
• 1st digit and 3rd digit of combination must equal the 3rd digit of 707.
• 0 cannot be used as you need 2, 3 digit numbers to make to combination.
• 2nd digit in all combinations must be the same,
we can find what numbers form 7 when added.

0+7
1+6
2+5
3+4

We know we can't use 0+7, so the other three combinations must be the 1st and 3rd digits.

Now we use a system of trial and error to find the 2nd digit.
0 is the only digit to fit with the answer equalling 707.

Therefore the 3 combinations must be:

304+403=707
502+205=707
601+106=707