Question

A car speeds up at 5m/s if the driver was already moving at 7m/s find his velocity after 3 seconds

Answer 1

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