a particle is projected horizontally with a speed u from the top of a plane inclined at an angle theta with the horizontal .how far from the point of projection will the particle strike the plane?(I have provided the attachment. Can anyone please explain how is it done. Thank You!)

Dear Student
We have to change the frame of reference. the new X axis is the plane parallel to inclined plane and Y axis is perpendicular to it. Therefore initial velocity along X axis is ucostheta and acceleration due to gravity is gsintheta. Rest is based on equations of motion separately for X axis and Y axis. changing reference makes the problem easy to solve.
Hope it helped