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12.]
[tex] 3^{2x + y} = \frac{1}{81} \\
3^{2x+y} = 3^{-4} [/tex]
2x + y = -4
y = -4 - 2x ... (i)

x² + 5y = 16 , Subt. persamaan (ii)
x² + 5(-4 - 2x) = 16
x² - 20 - 10x = 16
x² - 10x - 36 = 0

x₁ + x₂ = -b/a = 10
[A]

13.]
3³⁻ˣ + 3ˣ = 12
3³ / 3ˣ + 3ˣ = 12 [× 3ˣ]
27 + (3ˣ)² = 12 × 3ˣ
(3ˣ)² - (12 × 3ˣ) + 27 = 0
(3ˣ - 9)(3ˣ - 3) = 0
3ˣ = 9 atau 3ˣ = 3
x = 2 atau x = 1

x₂ - x₁ = 1 - 2 = -1
[C]

14.]
2ˣ⁺¹ ≥ (⅛)ˣ⁻³
2ˣ⁺¹ ≥ (2⁻³)ˣ⁻³
x + 1 ≥ -3x + 9
4x ≥ 8
x ≥ 2
[D]

15.]
[tex] (\frac{1}{27})^{x^2 - x} < (\frac19)^{x + 1} \\
(3^{-3(x^2-x)} < (3^{-2(x + 1)} [/tex]
-3(x² - x) < -2(x + 1)
-3x² + 3x < -2x - 2
-3x² + 5x + 2 < 0
3x² - 5x - 2 > 0
(3x + 1)(x - 2) > 0
x < - ⅓ atau x > 2
[C]