Answer: x^2
If we let u = x^2, then we can replace every copy of "x^2" with "u" like so
x^4 - 17x^2 + 16 = 0
(x^2)^2 - 17x^2 + 16 = 0
u^2 - 17u + 16 = 0 ... this is where the substitution takes place
From here you would use the quadratic formula or factoring to solve for u, getting two different solutions. Whatever those solutions may be, they are then plugged into u = x^2 to determine the four solutions for x.
