Answer: 20
Step-by-step explanation: Ok, to get the length of the diagonal we have to use the pythagorean theorem.
a² + b² = c²
10.4² + 17.1² = c²
108.16 + 292.41 = c²
400.57 = c²
c = 20.01
c = 20 (Rounded)
Therefore the answer is 20! I hope this helps!