$$21 - 3 ( a - 7 ) = a + 12$$
$a = \frac{15}{2} = 7\frac{1}{2} = 7.5$
$$21-3a+21=a+12$$
$$42-3a=a+12$$
$$42-3a-a=12$$
$$42-4a=12$$
$$-4a=12-42$$
$$-4a=-30$$
$$a=\frac{-30}{-4}$$
$$a=\frac{15}{2}$$
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