perbandingan akar2 persamaan xkuadrat + 4x + ( p-4) = 0 adalah 1 : 3 . nilai p?

diketahui
perbandingan akar2 pers kuadrat
x₁ : x₂
1 : 3
maka
3x₁ = x₂

x² + 4x + ( p-4) = 0 
a = 1
b = 4
c = (p - 4)

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x₁ + x₂ = -b/a = -4
x₁ + 3x₁ = -4
4x₁ = -4
x₁ = -1

x₂ = 3x₁
x₂ = 3(-1)
x₂ = -3

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x₁ . x₂ = c/a = (p - 4)
(-1)(-3) = p - 4
3 = p - 4
p = 7