a₁₄ - a₁₂ = 2d
d = (a₁₄ - a₁₂) / 2
d = (12 - 9) / 2 = 1.5
a₁₃ = a₁₂ + d
a₁₃ = 9 + 1.5 = 10.5
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a₁₄ - a₁₂ = 2d
d = (a₁₄ - a₁₂) / 2
d = (12 - 9) / 2 = 1.5
a₁₃ = a₁₂ + d
a₁₃ = 9 + 1.5 = 10.5