Two angles are complementary if the sum of their measures equals 90 degrees. Given the measures of the angles as \( (2x + 3)^\circ \) and \( 2x^\circ \), we can set up the equation:
\[ (2x + 3) + 2x = 90 \]
Now we can simplify and write the complete equation for the angle measures:
\[ (2x + 3)^\circ + (2x)^\circ = 90^\circ \]