If the angles \( (2x + 3)^\circ \) and \( 2x^\circ \) are complementary, they add up to \( 90^\circ \). Therefore, we can write the equation as follows:
\[ (2x + 3) + 2x = 90 \]
So the completed sentence is:
\[ (2x + 3)^\circ + 2x^\circ = 90^\circ \]