If the foot of the perpendicular from the point \( A(-1,4,3) \) on the plane \( P: 2x+my+nz=4 \) is \( \left(-2,\dfrac{7}{2},\dfrac{3}{2}\right) \), then the distance of the point \( A \) from the plane \( P \), measured parallel to a line with direction ratios \( 3,-1,-4 \), is equal to:
