Refer to the attached figure. We know that AB is 16 inches. We also know that AD=BD=10 and AE=BE=17
We're interested in DE, which we can compute as CD+CE
To compute CD, we can observe that ACD is a right triangle, and that AC is 8 inches long (it's half the base) and AD is 10 inches long (given).
So, by Pythagorean's theorem, we have
[tex] CD = \sqrt{10^2-8^2} = \sqrt{36} = 6 [/tex]
Similarly, we have
[tex] CE = \sqrt{17^2-8^2} = \sqrt{225} = 15 [/tex]
So, we have
[tex] DE=CD+CE=6+15=21 [/tex]