So far we have the equations: v = u + at and s = ½ (u+v) t
The second can be useful for finding displacement. However, if we don't know the final velocity, it's useless.
We could first work out v from "v = u + at". But let's be radical and substitute "v = u + at" into the second equation!
s = ½ (u + v) × t
A third equation, useful for finding displacement when you know the initial velocity, time and acceleration.