Arocket's acceleration is 6.0 m/s2. assuming it starts at 0 m/s, how long will it take for the rocket to reach a velocity of 42 m/s? 250 s 0.14 s 7.0 s 290 s

You said "A rocket's acceleration is 6.0 m/s2.".

That just means that its speed increases by 6 m/s every second.
Whenever you look at it, its speed is 6 m/s faster than it was
one second earlier.

If it starts out with zero speed, then its speed is 6 m/s after 1 second,
12 m/s after 2 seconds, 18 m/s after 3 seconds . . . etc.

How long does it take to reach 42 m/s ?

Well, how many times does it have to go 6 m/s FASTER 
in order to build up to 42 m/s ?

That's just (42/6) = 7 times.

Writing it correctly, with the units and everything, it looks like this:

(42 m/s) / (6 m/s²)

= (42/6)  (m/s) / (m/s²)

= (42/6)  (m/s · s²/m)

=  7 seconds