A car accelerates for 5 seconds at 2.1 m/s squared if the car intentionally has a speed of 10 m/s what is its new speed

Hey - for this problem you will find out your new velocity (similar to speed)

You take your Initial Velocity and add to it all the gained velocity you get during this process (which is the Acceleration multiplied by the amount of Time you are accelerating for)

So we get (Final Velocity) = (Initial Velocity) + (Time) (Acceleration)

In your equations, thats (Final Velocity) = (10 meters/second) + (5 seconds) (2.1 meters/second/second)

Which is 10 + 10.2 = 20.2 meters/second