Simplify the expression to a + bi form:

To simplify the expression \((−6−3i)−(12+10i)\), we will first distribute the negative sign through the terms in the second set of parentheses.

So we have:

\[ −6−3i−12−10i \]

Next, we can combine the real parts and the imaginary parts separately:

1. Real parts: \[ -6 - 12 = -18 \]

2. Imaginary parts: \[ -3i - 10i = -13i \]

Now, combining these results, we get:

\[ -18 - 13i \]

Thus, the expression in \( a + bi \) form is:

\[ \boxed{-18 - 13i} \]