The isosceles triangle divided the base in half and created 2 congruent right triangles.
The base of isosceles triangle = 8, so the leg of each right triangle = 8/2 = 4
The leg of isosceles triangle = 17. It's the hypo. of right triangle
so
x^2 = 17^2 - 4^2
x^2 = 289 - 16
x^2 = 273
x = √273
x = 16.5
Answer
Height of the isosceles triangle = 16.5