My try:
I assumed $$y_1=A\sin(\omega t - kx)$$ $$y_2=A\sin(\omega t - kx+\pi)$$
$$y_1+y_2=0$$
So how do I find $2\pi \Delta t/ T$ when y = 0 ? I don't know if I do it right or not ! Can you please help me out a little bit.
I think $$\pi = \frac{2\pi \Delta x}{\lambda}$$ which yields $\Delta x$ surely
Sorry to post this question here: I think mathematicians are great physicists also.