In the questions related to finding the sinusoidal equation of a wave with given snaphot how do we decide that whether the equation is y=Asin(wt-kx+phi) or y=Asin(kx-wt+phi)? It is given that i know that the wave travels in positive x direction.

Always take kx-wt. For a wave moving forward at t=0 and x=0 find the value of phi. The value of phi will be different for the two cases. Hence it will automatically get adjusted.NOTE: eg; Let us consider kx-wt when the other one is right. U can take -ve of it outside hence changing the value of A as well as phi. So dont worry it will get adjusted.