To solve the inequality
\[ \frac{a}{-2} < -9, \]
we can multiply both sides by -2. However, remember that when we multiply or divide by a negative number, we need to flip the inequality sign:
\[ a > -9 \cdot -2. \]
Calculating the right side:
\[ a > 18. \]
So we have the equivalent inequality:
\[ a > 18. \]
In terms of values written in order from least to greatest, the numbers that satisfy this inequality would be:
19, 20, 21, 22, ...
Thus, the answer is:
Equivalent Inequality: \( a > 18 \)