the 3rd kine eqns, p1

  • First two equations, with $v = v(t)$ and $x = x(t)$, are $$\tag{I1} v = v_0 + at, $$ $$\tag{I2} x = x_0 + v_0 t + \tfrac12 a t^2. $$
  • Turn (I1) into following $$\tag{I3} t = \frac{v - v_0}{a}. $$ and then substitute to (I2)