To solve for \( d \) in the equation \( 18d + 3d + 5d - 21d = 15 \), we first combine like terms on the left side:
\[ (18d + 3d + 5d - 21d) = (18 + 3 + 5 - 21)d = (26 - 21)d = 5d \]
Now the equation simplifies to:
\[ 5d = 15 \]
Next, we divide both sides by 5 to isolate \( d \):
\[ d = \frac{15}{5} = 3 \]
Thus, the solution is:
\[ \boxed{3} \]