Two angles are supplementary if the sum of their measures is \( 180^\circ \). Given the measures of the two angles as \( (2x + 5) \) and \( (x - 8) \), we can set up the equation as follows:
\[ (2x + 5) + (x - 8) = 180 \]
Now, let's simplify the equation:
\[ 2x + 5 + x - 8 = 180 \]
Combine like terms:
\[ 3x - 3 = 180 \]
This is the equation representing the relationship between the two supplementary angles.