For a given t_{1} and t_{2}, you found the average velocity over this time interval is given by
[tex]\bar{v} = v_0 + \frac{a(t_1+t_2)}{2}[/tex]
You want to set this to an expression for v(t), the instantaneous velocity at time t, and then solve for t.
You may want to verify your relevant equations again. You either have a typo above or you're not remembering the one you need correctly, which might be the cause for your confusion.
