We know that a soap film has two free surfaces, so total length of the film to be supported, $l = 2 \times 30 \text{ cm}$or $l = 60 \text{ cm} = 0.60 \text{ m}$Let T = surface tension of the film If F = total force on the slider due to surface tension, then$F = T \times 2l = T \times 0.6 \text{ N}$$W = 1.5 \times 10^{-2} \text{ N}$In equilibrium position, the force $F$ on the slider due to surface tension must be balanced by the weight $(W)$ supported by the slider.i.e. $F = W = m g$or $T \times 0.6 = 1.5 \times 10^{-2}$$\therefore T = \frac{1.5 \times 10^{-2}}{0.6}$$= 2.5 \times 10^{-2} \mathrm{Nm}^{-1}$