Gimana caranya tolong ya

Deret turun, maka b (beda) = U₂ - U₁ = 12 - 15 = -3
U₃₀ = U₁+(30-1)(b) = 15+(29)(-3) = 15 - 87 = -72
S₃₀ = 30/2(U₁+U₃₀) = 15(15+(-72)) = 15(-57) = -855