The plot of velocity versus time is a horizantal line between,(0, 4), and (5, 4). time is measures in seconds and velocity in meters per second. what is the position at t=5 s if the position at t=0 was 6 m?

Well, first of all, let's not make something complicated ... for which we're missing an important component anyway ... out of something simple.  There's more to "velocity" than 'meters per second'.  So let's just call that what it is: " Speed ".

Here's what you've told us:

-- Something is moving along a ruler or meter stick.

-- At every instant from time=0 to time=5, its speed was constant at 4 m/s.

-- At time=0, it was located at  6m .

We can calculate:

-- Moving steadily at 4 m/s for 5 seconds, it moved (4 x 5) = 20 meters.

-- If it was located at  6m  when time=0, then at time=6, it had advanced
to (6 + 20) = 26 meters.

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