The weight of Mr. Gerber and the plank is
660 + 40 = 700 N
To remain in equilibrium, the total weight should act at the center of mass of the plank, as shown in the figure.
Assume that the mass of the plank is uniformly distributed along its length.
The tension in each cable is T, therefore
2T = 700
T = 350 N
Answer: 350 N