Two blocks of masses $\mathrm{m}$ and $2 \mathrm{~m}$ kept on a frictionless horizontal surface are connected by a massless string. Two horizontal forces $F_1=(4.2 t) N$ and $F_2=(7.5 t) N$, where ' $t$ ' is time in seconds, are acting on the system as shown in the figure. The time at which the tension in the string between the two blocks becomes $10.6 \mathrm{~N}$ is s.