Explanation:
x + y = 17/18
x = 5/24
5/24 + y = 17/18
to bring both fractions to the same denominator (I see it is 72, but let's do it formally correct for you to learn), we look for the LCM, the lowest common multiple of both.
for this we split both numbers into their prime factors and then build the the combination of the longest chains per prime factor :
24 = 2×2×2×3
18 = 2×3×3
the LCM (and the new denominator) is then
2×2×2×3×3 = 8×9 = 72
so,
15/72 + y = 68/72
y = 53/72
53 is a prime number, so the fraction cannot be further refined.