Algebra homework: quadratic equation

Mr. S. asks:
How do you do this?
3^{2x}-5(3^x)+4=0

Solving:
it’s a quadratic equation! Replace 3^x with u.
u^2 - 5u + 4 = 0
(u - 4)(u - 1) = 0
u = 4
3^x = 4
x \log 3 = \log 4
x = \ log 4 / \log 3 or
u = 1
3^x = 1
x = 0

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