$$3(4s+1) = 21$$
Answer
$$ s = \frac{3}{2} $$
Explanation
$$ \begin{aligned} 3(4s+1) &= 21&& \text{simplify left side} \\[1 em]12s+3 &= 21&& \text{ move 3 to the right } \\[1 em]12s &= 21-3&& \\[1 em]12s &= 18&& \text{ divide both sides by $ 12 $ } \\[1 em]s &= \frac{18}{12}&& \\[1 em]s &= \frac{3}{2}&& \\[1 em] \end{aligned} $$
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