a)
Note that there are only two numbers that correspond complete wording of question: 59 and 68
It is easy to verify that the number found number is 68:
Look:
6 + 8 = 14 and
86 - 68 = 18
b)
Now you can solve one system:
x + y = 21
x - y = 5
Add both equations:
2x = 26
x = 13
So: y = 8