The progression given is an arithmetic progression (AP) with the first term \( a = 18 \) and a common difference \( d = 15 - 18 = -3 \).
To find the sum of the first \( n \) terms of an arithmetic progression, we use the formula:
\[ S_n = \frac{n}{2} \times (2a + (n-1)d) \]
Here, we want to find the sum \( S_{21} \) of the first 21 terms, so \( n = 21 \).
Substituting the values into the formula:
- \( a = 18 \)
- \( d = -3 \)
- \( n = 21 \)
So,
\[ S_{21} = \frac{21}{2} \times (2 \times 18 + (21 - 1) \times -3) \]
Calculating \( 2 \times 18 \):
\[ 2 \times 18 = 36 \]
Calculating \( (21 - 1) \times -3 \):
\[ (21 - 1) \times -3 = 20 \times -3 = -60 \]
Now we can substitute these results back into the formula:
\[ S_{21} = \frac{21}{2} \times (36 - 60) \]
Simplifying \( 36 - 60 \):
\[ 36 - 60 = -24 \]
Now substitute that back:
\[ S_{21} = \frac{21}{2} \times -24 \]
Calculating \( \frac{21}{2} \times -24 \):
\[ = \frac{21 \times -24}{2} = \frac{-504}{2} = -252 \]
Therefore, the sum of the first 21 terms of the progression is:
\[ \boxed{-252} \]