$$3x+12x+5 = 0$$
Answer
$$ x = -\frac{1}{3} $$
Explanation
$$ \begin{aligned} 3x+12x+5 &= 0&& \text{simplify left side} \\[1 em]15x+5 &= 0&& \text{ move the constants to the right } \\[1 em]15x &= -5&& \text{ divide both sides by $ 15 $ } \\[1 em]x &= -\frac{5}{15}&& \\[1 em]x &= -\frac{1}{3}&& \\[1 em] \end{aligned} $$
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