Since the two angles are complementary, their measures add up to 90°.
The measures of the angles are given as \( (2x + 3) \)° and \( 2x \)°. We can set up the equation as follows:
\[ (2x + 3) + 2x = 90 \]
So, filling in the blanks in your equation:
\[ (2x + 3)° + 2x° = 90° \]
Therefore, the completed equation is:
\[ (2x + 3)° + 2x° = 90° \]