Question
$$-7\cdot(4+r) = 21$$
Answer
$$ r = -7 $$
Explanation
$$ \begin{aligned} -7\cdot(4+r) &= 21&& \text{simplify left side} \\[1 em]-(28+7r) &= 21&& \\[1 em]-28-7r &= 21&& \\[1 em]-7r-28 &= 21&& \text{ move -28 to the right } \\[1 em]-7r &= 21+28&& \\[1 em]-7r &= 49&& \text{ divide both sides by $ -7 $ } \\[1 em]r &= -\frac{49}{7}&& \\[1 em]r &= -7&& \\[1 em] \end{aligned} $$
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