eliminate "vi" from the following equations 1)vf - vi + at S=vit+1/2at^2

eliminate "vi" from the following equations 1)vf - vi + at S=vit+1/2at^2

$\begin{array}{lrcl} (1) & v_f &=& v_i + a\cdot t \\ & v_i &=& v_f - a\cdot t \\\\ (2) & s &=& v_i \cdot t +\frac12 a\cdot t^2 \\ & s &=& (v_f - a\cdot t ) \cdot t +\frac12 a\cdot t^2 \\ & s &=& v_f\cdot t - a\cdot t \cdot t +\frac12 a\cdot t^2 \\ & s &=& v_f\cdot t - a\cdot t^2 +\frac12 a\cdot t^2 \\ & \mathbf{s} & \mathbf{=} & \mathbf{v_f\cdot t - \frac12 a\cdot t^2} \\ \end{array}$