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