I would try drawing the line on a coordinate plane to help.
5x-13=15+7x
-13=15+2x
-28=2x
-14=x
<u>Answer</u>
<em>The total displacement of the object is 320 m</em>
<u>Solution-</u>
From the formulas of mechanics,

Where,
s = displacement
u = initial velocity = 0 ( ∵ As the body was in rest in the beginning )
t = time taken = 8 s
a = acceleration = -10 m/s² ( ∵ -ve is because of the downward motion)
Putting all the values,
( ∵ -ve displacement means it is in downwards)
This one was though, but I think it might be y= -1/12x - 53/12.
Hope I helped!
Your answer is going to be 40