Car a start 10 meters ahead of car b. car a moves at v= (3.0i+0j) m/s and car b moves forward at v=(7.0i+0j)m/s how many seconds does it takes car b to catch up

Do both cars leave the starting line at the same time ?  That's kind of important.  I'll assume that they both start out at the same time, and now I'll proceed to answer the question that I have invented.

-- Car B moves 4 m/s faster than Car A .  

-- So Car-A's lead shrinks by 4 meters every second after they start.

-- It takes (10/4) = 2.5 seconds for Car-A's lead to shrink to zero.

-- So Car-B overtakes Car-A 2.5 seconds after they start.

it is a negative acceleration