we have that
If point C is a point of tangency
then
triangle ACD is a right triangle
so
Applying Pythagorean Theorem
DA^2=AC^2+CD^2
we have
AC=8 units
CD=15 units
substitute
DA^2=8^2+15^2
DA^2=64+225
DA^2=289
DA=17 units
Part 2
Find DE
we have that
DA=DE+EA ------> by addition postulate theorem
we have
DA=17 units
EA=AC=8 units
substitute
17=8+DE
DE=9 units