3 . 9^x + 9^(1-x) = 28. x1 + x2?

3.9ˣ + 9¹⁻ˣ = 28
3.9ˣ + 9¹.9⁻ˣ =28 ...kalikan 9ˣ
3. 9ˣ .9ˣ + 9. 9⁻ˣ.9ˣ = 28. 9ˣ 
3.9²ˣ + 9 - 28.9ˣ = 0
misal 9ˣ = a
3. a² - 28 a + 9 = 0
(3a - 1)(a  - 9) = 0
a = 1/3  atau a = 9
..
9ˣ = 1/3  atau 9ˣ = 9
3ˣ = 3⁻¹ atau x = 1
x = -1 atau x = 1
x₁ + x₂ = 0

solution by DB45