which Equations constant I use here ?

a truck covers a distance of 50 m in 8 s while smoothly sowing down to a final speed of 2.5 m/s . find its original speed

Dx = 50 m

t = 8 s

v = 2.5 m/s

Now I take this equation bit I don't now why I didn't get the correct answer

Dx = (v + v0 ) t/2

=50 = 2.5 + v0 ) 8 /2

=2 X 50 + 92.5 + v0 ) 8 /2

=100 = 20 + 8v0/2

=128v

v = -128