Since the two angles are complementary, their measures add up to 90 degrees. The measure of the first angle is \((2x + 3)^\circ\) and the measure of the second angle is \(2x^\circ\).
We can set up the equation as follows:
\[ (2x + 3) + 2x = 90 \]
This is the equation that represents the sum of the two angles being 90 degrees.