Since angles LMN and NOP are complementary angles, their measures add up to 90 degrees. Therefore, we can write the equation:
\[ m \angle LMN + m \angle NOP = 90 \]
Substituting the expressions for the angles, we have:
\[ 3x + (4x + 1) = 90 \]
Now, combine like terms:
\[ 3x + 4x + 1 = 90 \] \[ 7x + 1 = 90 \]
This is the simplified equation that will solve for \( x \).
So, the correct response is:
7x + 1 = 90.