Practice Final Exam - Page 706: 50

$-3i$

Removing the grouping symbols and then combining the real parts and the imaginary parts, the given expression, $ (12-6i)-(12-3i) ,$ simplifies to \begin{array}{l}\require{cancel} 12-6i-12+3i \\\\= (12-12)+(-6i+3i) \\\\= 0-3i \\\\= -3i .\end{array}