I've been having trouble sorting this one out. I need to compute the time it will take for a vehicle traveling along a straight line to reach a particular point. I have the initial velocity ($v_i$), acceleration ($a$), and distance ($dx$) to the point. I don't have the final velocity $v_f$ nor the time $t$ I've been trying to solve this equation for time ($t$) but that's where I'm stuck.
$ dx = v_it + {1\over2}at^2$