A soap film has two free surfaces, so the total length of the film to be supported,

$l=2\times 30\mathrm{ }\mathrm{cm}=0.60\mathrm{ }\mathrm{m}$

Let the surface tension of the film

If the total on the slider due to surface tension,

then $f=T\times 2l=T\times 0.6\mathrm{ }\mathrm{N}$

$W=1.5\times {10}^{-2} \mathrm{N}$

In equilibrium position, the force f on the slider due to surface tension must be balanced by the weight $\left(w\right)$ supported by the slider

i.e., $f=w=mg$

$T\times 0.6=1.5\times {10}^{-2}$

$T=\frac{1.5\times {10}^{-2}}{0.6}$

$T=2.5\times {10}^{-2} \mathrm{N} {\mathrm{m}}^{-1}$