Plz i really need this calculate the speed of an 7.0×10^4 kg airliner with a kinetic energy of 1.4×10^9 j. answer in units of m/s

I have a hunch that you've got the question totally covered, and what you actually need is the answer.

Multiply each side by 2 :  2 x KE = (mass) ( speed²)

Divide each side by (mass):  Speed² = 2 x KE / mass

Square root each side:   Speed = √(2 KE/mass)

Look at that !  The question GIVES you the KE and the mass.  All you have to do is plug those 2 numbers into the right side of that equation, turn the crank, do the arithmetic, and the speed falls out.

I get 200 m/s .  You need to check my work.

(IF that's correct or anywhere close, it's equivalent to something around 447 miles per hour, which is very reasonable for a cruising airliner.)

this is a good question! i'll do some research and come back when i have the answer!