A car speeds up at 5m/s if the driver was already moving at 7m/s find his velocity after 3 seconds
I assume you mean it speeds up at 5m/s^2
That means that each second the speed increases by 5m/s
So, after 3 seconds, it has increased by 15m/s to 22m/s
or, more conventionally,
V = V0 + at = 7 + 5*3 = 22