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Because this is a fourth degree that will be tricky to factor as it is, we will do a u substitution. Let . We can now rewrite that polynomial in terms of u:
. Filling into the quadratic formula we have
. Simplifying down
and
or
. That means that u = 8 or u = -1. But don't forget that we let
, so we have to put x-squared back in for u. That gives us
which simplifies down to
. That also gives us
. When we take the square root of -1, we have to use the fact that -1 = i^2, so we sub that in to get
. All in all, your solutions are as follows:
which is choice b.