$$C=21 \times 3.4+2.1 \times 5.4-0.8 \times 21$$
$C=65.94$
$$C=71.4+2.1\times 5.4-0.8\times 21$$
$$C=71.4+11.34-0.8\times 21$$
$$C=82.74-0.8\times 21$$
$$C=82.74-16.8$$
$$C=65.94$$
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$C≔65.94$