the initial line appears to be vertical -hence the X value doesn't change, therefore the equation is x='constant' . You can get the value of the constant from the graph (NOTE - if this is intended to be the flight of a aircraft then it is evidently impossible to climb like that - ie it gains altitiude in 0 time - an infinite rate of climb. Note 2 - it is not the whole line that is valid for the climb - it is a 'line segment' and is bounded by y=0 and y = 9
also - the numbers on your graph don't seem to make sense: you have -10 at both ends of the x axis you have what looks like 1/5 halfway along the negative axis the path hits the ground twice where did this drawing originate?
OK so the x axis is not time - it is distance? and y axis is height? it does mean that you have a truly vertical lift at the beginning - most rides climb up a sloping track.