I hope this picture helps you!
Using the Pythagorean theorem a^2 +b^2 = c^2, where a and b are the sides of a triangle and c is the hypotenuse.
BA and AC are sides and BC is the hypotenuse.
we have 23^2 + b^2 = 45^2
529 + b^2 = 2025
b^2 = 2025 - 529
b^2 = 1496
b = sqrt(1496)
b = 38.68 = 38.7
The length of AC = 38.7
If you want me to solve for "Y" then, the answer will be,

ir if you want me to solve for "X" instead then the answer will be,

18.84
Since the formula of circumference is C = 2 pi r or C = pi times diameter, you can conclude that the circumference is 18.84.
Assuming that pi is 3.14 in this case rather than the full pi or just in terms of pi.
Hope this helps!