From equation 7 to equation 8

Expand the following: \begin{align} x(t)= \frac{F_b}{2m}t^2+v_0t =&\cssId{Step1}{0=-\frac{F_b}{m}t_s+v_0}\\[3px] &\cssId{Step2}{t_s= \frac{mv_0}{F_b}}\\[3px] \end{align}

1. In the first step we see the equation give us a result of 0
2. In the second step we calculate the value of time, previously introduced