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what is 2^ 2x+1 -17 x 2^x +8=0

it says in the answer that x=-1 or 3 but could anyone tell me how to get to that? thanks
Let 2^x = y
2y^2 -17y + 8 =0
Factorise
(2y-1)(y-8)=0
y=0.5,8
So 2^x = 0.5,8
x=-1,3

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