Given:
Xo = 8.1 m @ t = 0 seconds
V = -1.6 m/s (constant velocity)
Solution
Start with the definition of velocity
V = dx/dt
Separate variable
Vdt = dx
Integrate both sides
Velocity is constant so we can pull it out of the integral
V ∫dt = ∫dx
Apply boundary conditions
V (t-0) = X - Xo
X = Vt + Xo
X = -1.6t + 8.1 (units: m)