To evaluate the expression \( 2i - 18i \), we can combine the like terms:
\[ 2i - 18i = (2 - 18)i = -16i \]
In the form \( a + bi \), where \( a \) is the real part and \( b \) is the imaginary part, we can express \(-16i\) as \( 0 - 16i \).
Thus, the answer is:
\[ 0 - 16i \]
or simply:
\[ 0 + (-16)i \]
In conclusion, the final answer is:
\[ 0 - 16i \]