Explanation:
first we need to find the length of CD, so that we can use Pythagoras to find AC.
to get CD we use again Pythagoras. there is a smaller right-angled triangle on top of the BCD rectangle.
so, AD = 11 = 4 + 7
7 = the vertical leg of the upper right-angled triangle.
the horizontal leg is congruent to CD and is
sqrt(16² - 7²) = sqrt(256 - 49) = sqrt(207).
AC² = sqrt(207)² + 11² = 207 + 121 = 328
AC = sqrt(328) = 18.11077028... cm ≈ 18.1 cm