The inverse of $ z $ is:
$$ z^{-1} = -\frac{ 1 }{ 39 }-\frac{ 1 }{ 26 }i $$We will denote the inverse of $ z $ as $ z_1 $. The inverse can be found in three steps.
Step 1: Rewrite complex number as its reciprocal
$$ z_1 = \frac{1}{ -12+18i } $$Step 2: Multiply top and bottom by complex conjugate of $ z $
$$ z_1 = \frac{1}{ -12+18i } \cdot \frac{ -12-18i }{ -12-18i } $$Step 3: Simplify
$$ z_1 = \frac{ -12-18i }{ 468 } $$$$ z_1 = \frac{ -12 }{ 468 } - \frac{ 18 }{ 468 } \cdot i$$$$ z_1 = -\frac{ 1 }{ 39 }-\frac{ 1 }{ 26 }i $$