Answer:
Third choice is the correct one
Explanation:
We are asked to use the u substitution. Therefore, we will assume that:
u = x^2
Substitute with u in the given expression as follows:
9x^4 - 2x^2 - 7 = 0
9u^2 - 2u - 7 = 0
(u-1)(9u+7) = 0
either u = 1
or u = -7/9
Now, u = x^2, this means that:
at u = 1:
x^2 = 1
x = ± √1 = ±1
at u = -7/9
x = ±i√7/9 (the i indicates the imaginary value)
Hope this helps :)
