Two opposite forces F_{1} = 120N and F_{2} = 80N act on an heavy elastic plank of modulus of elasticity y = 2×10^{11} N/m^{2} and length L = 1m placed over a smooth horizontal surface. The cross-sectional area of plank is A = 0.5m^{2} . If the change in the length of plank is x × 10^{–9}m, then find x ?