[THOUGHT] Let me solve this step by step.

1. First, I need to find the direction vector of line $L_1$ (intersection of the two planes).
2. Then find line $L_2$ passing through $P(2, -1, 3)$ parallel to $L_1$.
3. Find point $Q$ where $L_2$ meets plane $\Pi: 2x + y - 2z = 6$.
4. Find point $R$, the foot of perpendicular from $P$ to plane $\Pi$.
5. Calculate $\ell = |PQ|$ and $A =$ area of triangle $PQR$.
6. Finally compute $4\ell^2 A^2$.

Let me use Python to do these calculations. [/THOUGHT] [PYTHON] import numpy as np from numpy.linalg import norm

Plane 1: 2x + 3y + z = 4, normal n1 = (2, 3, 1)

Plane 2: x + 2y + z = 5, normal n2 = (1, 2, 1)

n1 = np.array([2, 3, 1]) n2 = np.array([1, 2, 1])

Direction of L1 is n1 × n2

d1 = np.cross(n1, n2) print("Direction of L1:", d1)

L2 passes through P(2, -1, 3) with direction d1

P = np.array([2, -1, 3])

Parametric form of L2: (2 + d1[0]*t, -1 + d1[1]*t, 3 + d1[2]*t)

Plane Pi: 2x + y - 2z = 6

Substitute L2 into Pi:

2(2 + d1[0]*t) + (-1 + d1[1]*t) - 2(3 + d1[2]*t) = 6

4 + 2*d1[0]*t - 1 + d1[1]t - 6 - 2d1[2]*t = 6

(2d1[0] + d1[1] - 2d1[2])*t + (4 - 1 - 6) = 6

(2d1[0] + d1[1] - 2d1[2])*t - 3 = 6

(2d1[0] + d1[1] - 2d1[2])*t = 9

coeff_t = 2d1[0] + d1[1] - 2d1[2] print("Coefficient of t:", coeff_t)

t_Q = 9 / coeff_t print("t for Q:", t_Q)

Q = P + t_Q * d1 print("Point Q:", Q)

Length PQ

PQ = Q - P ell = norm(PQ) print("Length PQ (ell):", ell)

Find R: foot of perpendicular from P to plane Pi: 2x + y - 2z = 6

Normal to Pi: n_pi = (2, 1, -2)

n_pi = np.array([2, 1, -2])

Line from P perpendicular to Pi: P + s * n_pi

Substitute into plane equation:

2(2 + 2s) + (-1 + s) - 2(3 - 2s) = 6

4 + 4s - 1 + s - 6 + 4s = 6

9s - 3 = 6

9s = 9

s = 1

s_R = (6 - (2P[0] + P[1] - 2P[2])) / (2n_pi[0] + n_pi[1] - 2n_pi[2]) print("s for R:", s_R)

R = P + s_R * n_pi print("Point R:", R)

Verify R is on plane Pi

check_R = 2R[0] + R[1] - 2R[2] print("Check R on Pi (should be 6):", check_R)

Distance PR (perpendicular distance from P to plane)

PR = R - P dist_PR = norm(PR) print("Distance PR:", dist_PR)

Area of triangle PQR

Using cross product: Area = 0.5 * |PQ × PR|

cross_PQ_PR = np.cross(PQ, PR) A = 0.5 * norm(cross_PQ_PR) print("Area A:", A)

Calculate 4 * ell^2 * A^2

result = 4 * ell2 * A2 print("4 * ell^2 * A^2:", result) [/PYTHON]

[THOUGHT] The Python calculation gives us:

• Length $\ell = |PQ| = 9\sqrt{3}$ (approximately 15.588)
• Area $A$ of triangle $PQR$ (approximately 22.946)
• $4\ell^2 A^2 = 511758$

The result is exactly 511758. [/THOUGHT]

\boxed{511758}