To solve this problem, we start from the definition of complementary angles. Two angles are complementary if the sum of their measures is 90 degrees.
We have two angles:
- \( 3x^\circ \)
- \( (4x + 1)^\circ \)
Since these angles are complementary, we can write the equation:
\[ 3x + (4x + 1) = 90 \]
Now, simplify this equation:
\[ 3x + 4x + 1 = 90 \] \[ 7x + 1 = 90 \]
The simplified equation is \( 7x + 1 = 90 \).
So, the correct answer is:
D. (7x + 1)° = 90°